@phdthesis{Franke1995, author = {Franke, Fabian}, title = {Produktion und Zerfall von Neutralinos im Nichtminimalen Supersymmetrischen Standardmodell}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-25666}, school = {Universit{\"a}t W{\"u}rzburg}, year = {1995}, abstract = {Das Ziel der vorliegenden Arbeit ist eine umfassende Analyse von Erzeugung und anschließenden Zerf{\"a}llen von Neutralinos im Nichtminimalen Supersymmetrischen Standardmodell (NMSSM) speziell f{\"u}r den n{\"a}chsten verf{\"u}gbaren Elektron-Positron-Speicherring LEP2 am CERN mit einer voraussichtlichen Schwerpunktsenergie von 190 GeV. Das NMSSM ist die einfachste Erweiterung des Minimalen Supersymmetrischen Standardmodells MSSM mit einem Singlett-Superfeld, so dass der Higgs-Sektor insgesamt sieben physikalische Higgs-Teilchen enth{\"a}lt, und zwar drei neutrale skalare, zwei pseudoskalare und zwei geladene. Weiterhin enth{\"a}lt das NMSSM f{\"u}nf Neutralinos gegen{\"u}ber vier im MSSM. In dieser Arbeit pr{\"a}sentieren wir die 5 x 5 Neutralinomischungsmatrix, stellen die Eigenwertgleichung auf und analysieren das Massenspektrum und die Parameterabh{\"a}ngigkeit m{\"o}glicher masseloser Zust{\"a}nde. F{\"u}r die Untersuchung von Neutralinoproduktion und -zerfall wurden verschiedene Szenarien gew{\"a}hlt, in denen das leichteste Neutralino eine Masse von 10 GeV und eine Singlettkomponente von {\"u}ber 90\% besitzt oder in denen das leichteste Neutralino bis zu 50 Gev schwer ist und sich der Singlettanteil auf die beiden leichtesten Neutralinos verteilt. Die Wirkungsquerschnitte f{\"u}r die Neutralinoproduktion wurden in den gew{\"a}hlten Szenarien f{\"u}r Schwerpunktsenergien von 100 GeV bis 600 GeV berechnet, also bis zu einem Bereich, den ein geplanter Elektron-Positron-Linearbeschleuniger erreichen kann. Typische Wirkungsquerschnitte f{\"u}r die direkte Produktion vorwiegend singlettartiger Neutralinos liegen im Bereich von 100 fb. Selbst wenn das leichteste Neutralino sehr leicht ist, kann das n{\"a}chste bereits so schwer sein, dass bei LEP2 nur die nicht nachtweisbare Paarproduktion des leichtesten supersymmetrischen Teilchens m{\"o}glich ist. Somit ist bei LEP2 keine Erh{\"o}hung der unteren Neutralinomassengrenzen im NMSSM zu erwarten, falls kein Neutralino gefunden wird. In Szenarien mit leichten singlettartigen Neutralinos k{\"o}nnen sehr oft auch sehr leichte Higgs-Bosonen mit Massen unterhalb der im MSSM vorhandenen Grenzen existieren. Somit kann in allen unseren Szenarien der Neutralinozerfall in ein skalares oder pseudoskalares Higgs-Boson m{\"o}glich sein und dann Verweigungsverh{\"a}ltnisse bis zu fast 100\% erreichen. Wir berechnen in dieser Arbeit f{\"u}r die bei LEP2 produzierbaren Neutralinos die Verwzeigungsverh{\"a}ltnisse f{\"u}r die Zweik{\"o}rperzerf{\"a}lle in Higgs-Bosonen, die Dreik{\"o}rperzerf{\"a}lle in zwei Fermionen und den Schleifenzerfall in ein Photon. In allen F{\"a}llen befindet sich im Endzustand außerdem das unsichtbare leichteste Neutralino, dass sich experimentell als fehlende Energie niederschl{\"a}gt. Zur Bestimmung der Signaturen betrachten wir außerdem die anschließenden Zerfallsmodi der leichten Higgs-Bosonen. Der Nachweis von leichten singlettartigen Neutralinos im NMSSM kann einerseits unm{\"o}glich sein, wenn entweder die schweren Neutralinos bei der verf{\"u}gbaren Schwerpunktsenergie nicht produziert werden k{\"o}nnen oder {\"u}ber Higgs-Bosonen vollkommen in das LSP zerfallen, andererseits aber auch durch klare Signaturen mit einem Photon oder mit Jets im Endzustand erleichtert werden. Bei LEP2 sollten also durchaus Chancen bestehen, auch im Rahmen des NMSSM ein Neutralino zu entdecken. Zumindest werden sich weitere Einschr{\"a}nkungen des Parameterraums ergeben. Der Dissertation ist ein Anhang beigef{\"u}gt, der eine vollst{\"a}ndige Liste aller Feynman-Regeln des NMSSM enth{\"a}lt, die sich von denjenigen des MSSM unterscheiden.}, subject = {Supersymmetrie}, language = {de} } @phdthesis{Voelker2003, author = {V{\"o}lker, Roland}, title = {Staubzerst{\"o}rung durch interstellare Stoßfronten}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-7707}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2003}, abstract = {Ein Teil der interstellaren Materie (ISM) liegt in Form von winzigen Festk{\"o}rpern vor, die mit dem interstellaren Gas vermischt sind. Diese Teilchen werden als interstellarer Staub bezeichnet. Obwohl der Staubanteil an der Gesamtmasse der ISM nur etwa 1\% betr{\"a}gt, kann sein Einfluß auf das interstellare Strahlungsfeld und die Dynamik des Gases nicht vernachl{\"a}ssigt werden. So ist er die Hauptursache f{\"u}r Extinktion, Streuung und Polarisation von Licht. Außerdem stellt der Staub ein wichtiges K{\"u}hlmittel f{\"u}r das interstellare Medium dar und beeinflußt die chemischen Prozesse innerhalb der ISM. Staubpartikel unterliegen Wachstums- und Zerst{\"o}rungsprozessen. So k{\"o}nnen sie Molek{\"u}le aus der Umgebung an ihrer Oberfl{\"a}che anlagern (Akkretion) oder sich mit anderen Partikeln zu gr{\"o}ßeren Staubteilchen verbinden (Koagulation). Durch die Wechselwirkung mit Ionen kann Oberfl{\"a}chenmaterial abgetragen werden (Sputtering) und das Kollidieren von Staubpartikeln f{\"u}hrt zu deren Zerschlagung in kleinere Teilchen oder (Shattering) deren Vaporisation. Außerdem sind Staubpartikel an das Gas gekoppelt und werden von diesem mitgerissen. Der Schwerpunkt der Vorliegenden Arbeit war die Untersuchung der dynamischen Prozesse, denen Staubpartikel bei der Durchquerung von interstellaren Stoßfronten unterworfen sind. In diesem Zusammenhang spielen vorallem die destruktiven Prozesse und die Kopplung an das Gas eine wichtige Rolle. Es wurden Gleichungen eingef{\"u}hrt, die die {\"A}nderung einer Staubverteilung durch diese Vorg{\"a}nge beschreiben. Im Gegensatz zu bisherigen Modellen werden die Staubteilchen darin nicht allein durch ihre Masse, sondern auch durch ihre Geschwindigkeit charakterisiert. Auf diese Weise kann die Impulserhaltung bei einer Partikelkollision gew{\"a}hrleistet werden und es ist beispielsweise m{\"o}glich auch St{\"o}ße gleich schwerer Partikel zu beschreiben. Die Gleichungen der Staub- und Hydrodynamik wurden f{\"u}r den Fall von station{\"a}ren, eindimensionalen Stoßwellen numerisch gel{\"o}st, wobei die Wechselwirkungen zwischen Gas und Staub ber{\"u}cksichtigt wurden. Mit Hilfe des Modells wurden die Wirkung verschieden starker Stoßwellen auf eine Staubverteilung untersucht. Dabei wurden verschiedene Staubmaterialien zugrunde gelegt.}, subject = {Interstellarer Staub}, language = {de} } @phdthesis{Jacobs2003, author = {Jacobs, Arne}, title = {Andreev-Streuung, Josephson-Bloch-Oszillationen und Zener-Tunneln in Heterokontakten aus Normal- und Supraleitern}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-9237}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2003}, abstract = {Die vorliegende Arbeit beleuchtet verschiedene Aspekte des Ladungstransports in Heterokontakten aus Normal- (N) und Supraleitern (S) im Rahmen des Bogoliubov-de Gennes-Formalismus. Dabei ist der bestimmende Prozeß die Andreev-Streuung: die Streuung von Elektronen in L{\"o}cher, bzw. umgekehrt, an r{\"a}umlichen Variationen des supraleitenden Paarpotentials unter Erzeugung, bzw. Vernichtung, eines Cooperpaares und damit der Induktion eines Suprastroms. Befindet sich ein Supraleiter zwischen zwei normalleitenden Bereichen, so wandelt sich der an der einen NS-Phasengrenze durch Andreev-Streuung induzierte Suprastrom an der anderen NS-Phasengrenze wieder in einen durch Quasiteilchen getragenen Strom um. Diese Umwandlung erfolgt durch den Einfall eines Quasiteilchens, dessen Charakter dem des auf der gegen{\"u}berliegenden Seite des Supraleiters einfallenden Quasiteilchens entgegengerichtet ist, wie anhand von Wellenpaket-Rechnungen explizit gezeigt wird. Ersetzt man den Supraleiter durch einen mesoskopischen SNS-Kontakt, ist die Vielteilchen-Konfiguration in der mittleren N-Schicht phasenkoh{\"a}rent und daher verschieden von den unkorrelierten Quasiteilchen-Anregungen, die die verschobene Fermi-Kugel in den normalleitenden Zuleitungen bilden. Die Josephson-Str{\"o}me, die durch die Quasiteilchen in der mittleren N-Schicht getragen werden, werden unter zwei verschiedenen Modellannahmen berechnet: Im einen Fall werden nur Streuzust{\"a}nde als Startzust{\"a}nde betrachtet, im anderen, bei gleichzeitiger Ber{\"u}cksichtigung eines normalstreuenden Potentials, nur gebundene Zust{\"a}nde. Der SNS-Kontakt wird durch eine supraleitend/halbleitende Heterostruktur modelliert, deren Parameter-Werte sich an den Experimenten der Gruppe von Herbert Kroemer in Santa Barbara orientieren. Wenn die supraleitenden Bereiche ohne normalleitende Zuleitungen direkt mit einem Reservoir von Cooperpaaren verbunden sind, fallen nur Quasiteilchen in Streuzust{\"a}nden aus den supraleitenden B{\"a}nken auf die NS-Phasengrenzen des Kontaktes ein. Mit den Normalleiter-Wellenfunktionen, die sich bei Anlegen einer Spannung V aus diesen Startzust{\"a}nden entwickeln, wird die Josephson-Wechselstromdichte in der Mitte der N-Schicht bei der Temperatur T = 2,2 K berechnet. Die Stromdichte weist spannungsabh{\"a}ngige Oszillationen in der Zeit auf, deren Periode das Inverse der Josephson-Frequenz ist. Alle Stromdichten zeigen bei kleinen Spannungen einen steilen Anstieg ihres Betrages, der durch Quasiteilchen zustandekommt, die durch das elektrische Feld aus dem Kondensat kommend in den Paarpotentialtopf hineingezogen werden und dort bei kleinen Spannungen eine große Zahl von Andreev-Streuungen erfahren, wobei sie bei jedem Elektron-Loch-Zyklus die Ladung 2e durch die N-Schicht transportieren. Im zweiten betrachteten Fall wird unter Ber{\"u}cksichtigung von Normalstreuung der Gesamtzustand des Systems zu jedem Zeitpunkt durch eine Superposition von gebundenen Zust{\"a}nden ausgedr{\"u}ckt. Die Energie dieser gebundenen Zust{\"a}nde ist abh{\"a}ngig von der Phasendifferenz Phi zwischen den supraleitenden Schichten. F{\"u}r Werte der Phasendifferenz von ganzzahligen Vielfachen von Pi sind Zust{\"a}nde entgegengerichteter Impulse paarweise entartet. Das normalstreuende Potential mischt diese Zust{\"a}nde, hebt ihre Entartung auf und f{\"u}hrt zu Energiel{\"u}cken: Es bilden sich Energieb{\"a}nder im Phi-Raum, die formal den Bloch-B{\"a}ndern von Kristallen im Wellenzahlraum entsprechen. Wird eine {\"a}ußere Spannung angelegt, so {\"a}ndert sich die Phasendifferenz gem{\"a}ß der Josephson-Gleichung mit der Zeit und die Quasiteilchen oszillieren in ihren jeweiligen Phi-Bloch-B{\"a}ndern: Diese Josephson-Bloch-Oszillationen ergeben den "normalen" Josephson-Wechselstrom, der zwischen positiven und negativen Werten schwingt und im zeitlichen Mittel Null ist. Zus{\"a}tzlich k{\"o}nnen die Quasiteilchen durch Zener-Tunneln --- wie der analoge Prozeß in der Halbleiterphysik genannt wird --- in h{\"o}here B{\"a}nder {\"u}bergehen. W{\"a}hrend sich die Richtung der Josephson-Stromdichte zu den Zeiten minimaler Energiel{\"u}cke umkehrt, hat die Zener-Tunnel-Stromdichte nach einem Tunnel-Prozeß das gleiche Vorzeichen, das die Josephson-Stromdichte vor dem Tunnel-Prozeß hatte. Wenn die angelegte Spannung hinreichend groß ist und gen{\"u}gend Quasiteilchen in das h{\"o}here Band tunneln, {\"u}berkompensiert die Zener-Tunnel-Stromdichte in der Halbperiode nach dem Tunnel-Prozeß die Josephson-Stromdichte, und die Gesamtstromdichte schwingt wieder in dieselbe Richtung wie vor dem Zener-Tunneln. Somit hat sich gewissermaßen die Periode halbiert: Die Gesamtstromdichte schwingt mit der doppelten Josephson-Frequenz. Allen untersuchten Aspekten des Ladungstransports durch Heterokontakte aus Normal- und Supraleitern ist eines gemein: Der f{\"u}r ihr Verst{\"a}ndnis fundamentale Prozeß ist die Andreev-Streuung.}, subject = {Supraleiter}, language = {de} } @phdthesis{Kneiske2004, author = {Kneiske, Tanja}, title = {Wechselwirkung von Gammastrahlung mit dem metagalaktischen Strahlungsfeld}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-11479}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2004}, abstract = {Im Rahmen dieser Arbeit wurden Effekte der Paarbildung durch Wechselwirkung von hochenergetischer Gammastrahlung mit dem metagalaktischen FIR-UV Strahlungsfeld (MRF) untersucht. Einerseits hat die Paarbildung Folgen f"ur die beobachteten Spektren aktiver Galaxienkerne, andererseits hat sie auch einen gro"sen Einflu"s auf den extragalaktischen Gammastrahlungshintergrund. Es wurde ein verbesserte Version f"ur das Modell des FIR-UV Strahlungsfelds vorgestellt, mit dessen Hilfe aus beobachteten Daten intrinsische Blazarspektren ermittelt wurden. Im weiteren wurde ein auf EGRET-Blazaren basierendes Modell f"ur den Gammastrahlungshintergrund berechnet, in dem besonderer Wert auf die korrekte Beschreibung der Absorption prim"arer und der daraus resultierenden sekund"aren Gammastrahlung gelegt wurde. Schlie"slich wurde gezeigt, da"s der Beitrag von BL Lac Objekten zum Gammahintergrund nicht nur der fehlende Flu"s, sondern auch die spektrale Form der aus EGRET Beobachtungen gewonnenen Daten erkl"art werden kann, ohne den gegenw"artigen TeV-Daten zu widersprechen.}, subject = {Metagalaxis}, language = {de} } @phdthesis{Bechmann2004, author = {Bechmann, Michael}, title = {Dynamics in quantum spin glass systems}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-12519}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2004}, abstract = {This thesis aims at a description of the equilibrium dynamics of quantum spin glass systems. To this end a generic fermionic SU(2), spin 1/2 spin glass model with infinite-range interactions is defined in the first part. The model is treated in the framework of imaginary-time Grassmann field theory along with the replica formalism. A dynamical two-step decoupling procedure, which retains the full time dependence of the (replica-symmetric) saddle point, is presented. As a main result, a set of highly coupled self-consistency equations for the spin-spin correlations can be formulated. Beyond the so-called spin-static approximation two complementary systematic approximation schemes are developed in order to render the occurring integration problem feasible. One of these methods restricts the quantum-spin dynamics to a manageable number of bosonic Matsubara frequencies. A sequence of improved approximants to some quantity can be obtained by gradually extending the set of employed discrete frequencies. Extrapolation of such a sequence yields an estimate of the full dynamical solution. The other method is based on a perturbative expansion of the self-consistency equations in terms of the dynamical correlations. In the second part these techniques are applied to the isotropic Heisenberg spin glass both on the Fock space (HSGF) and, exploiting the Popov-Fedotov trick, on the spin space (HSGS). The critical temperatures of the paramagnet to spin glass phase transitions are determined accurately. Compared to the spin-static results, the dynamics causes slight increases of T_c by about 3\% and 2\%, respectively. For the HSGS the specific heat C(T) is investigated in the paramagnetic phase and, by way of a perturbative method, below but close to T_c. The exact C(T)-curve is shown to exhibit a pronounced non-analyticity at T_c and, contradictory to recent reports by other authors, there is no indication of maximum above T_c. In the last part of this thesis the spin glass model is augmented with a nearest-neighbor hopping term on an infinite-dimensional cubic lattice. An extended self-consistency structure can be derived by combining the decoupling procedure with the dynamical CPA method. For the itinerant Ising spin glass numerous solutions within the spin-static approximation are presented both at finite and zero temperature. Systematic dynamical corrections to the spin-static phase diagram in the plane of temperature and hopping strength are calculated, and the location of the quantum critical point is determined.}, subject = {Spinglas}, language = {en} } @phdthesis{Eckl2004, author = {Eckl, Thomas}, title = {Phenomenological phase-fluctuation model for the underdoped cuprates}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-12115}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2004}, abstract = {In this thesis, a phenomenological phase-fluctuation model for the pseudogap regime of the underdoped cuprates was discussed. The key idea of the phase-fluctuation scenario in the high-T_c superconductors is the notion that the pseudogap observed in a wide variety of experiments arises from phase fluctuations of the superconducting gap. In this scenario, below a mean-field temperature scale T_c^{MF}, a d_{x^2-y^2}-wave gap amplitude is assumed to develop. However, the superconducting transition is suppressed to a considerably lower transition temperature T_c by phase fluctuations. In the intermediate temperature regime between T_c^{MF} and T_c, phase fluctuations of the superconducting order parameter give rise to the pseudogap phenomena. The phenomenological phase-fluctuation model discussed in this thesis consists of a two-dimensional BCS-like Hamiltonian where the phase of the pairing-amplitude is free to fluctuate. The fluctuations of the phase were treated by a Monte Carlo simulation of a classical XY model. First, the density of states was calculated. The quasiparticle tunneling conductance (dI/dV) obtained from our phenomenological phase fluctuation model was able to reproduce characteristic and salient features of recent scanning-tunneling studies of Bi2212 and Bi2201 suggesting that the pseudogap behavior observed in these experiments arises from phase fluctuations of the d_{x^2-y^2}-wave pairing gap. In calculating the single-particle spectral weight, we were further able to show how phase fluctuations influence the experimentally observed quasiparticle spectra in detail. In particular the disappearance of the BCS-Bogoliubov quasiparticle band at T_c and the change from a more V-like superconducting gap to a rather U-like pseudogap above T_c can be explained in a consistent way by assuming that the low-energy pseudogap in the underdoped cuprates is due to phase fluctuations of a local d_{x^2-y^2}-wave pairing gap with fixed magnitude. Furthermore, phase fluctuations can explain why the pseudogap starts closing from the nodal points, whereas it rather fills in along the anti-nodal directions and they can also account for the characteristic temperature dependence of the superconducting (pi,0)-photoemission-peak. Next, we have shown that the "violation" of the low-frequency optical sum rule recently observed in the SC state of underdoped Bi2212, which is associated with a reduction of kinetic energy, can be related to the role of phase fluctuations. The decrease in kinetic energy is due to the sharpening of the quasiparticle peaks close to the superconducting transition at T_c == T_{KT}, where the phase correlation length xi diverges. A detailed analysis of the temperature and frequency dependence of the optical conductivity sigma(omega)=sigma_1(omega)+i sigma_2(omega) revealed a superconducting scaling of sigma_2(omega), which starts already above T_c, exactly as observed in high-frequency microwave conductivity experiments on Bi2212. On the other hand, our model was only able to account for the characteristic peak, which is observed in sigma_1(omega) close to the superconducting transition, after the inclusion of an additional marginal-Fermi-liquid scattering-rate in the optical conductivity formula. Finally, we calculated the static uniform diamagnetic susceptibility. It turned out that the precursor effects of the fluctuating diamagnetism above T_c are very small and limited to temperatures close to T_c in a phase-fluctuation scenario of the pseudogap. Instead, the temperature dependence of the uniform static magnetic susceptibility is dominated by the Pauli spin susceptibility, which displayed a very characteristic temperature dependence, independent of the details of the gap function used in our model. This temperature dependence is qualitatively very similar to the experimentally observed change of the Knight-shift as a function of temperature in underdoped Bi2212.}, subject = {Hochtemperatursupraleiter}, language = {en} } @phdthesis{Deppisch2004, author = {Deppisch, Frank}, title = {Towards a reconstruction of the SUSY seesaw model}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-12757}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2004}, abstract = {In this work, we studied in great detail how the unknown parameters of the SUSY seesaw model can be determined from measurements of observables at or below collider energies, namely rare flavor violating decays of leptons, slepton pair production processes at linear colliders and slepton mass differences. This is a challenging task as there is an intricate dependence of the observables on the unknown seesaw, light neutrino and mSUGRA parameters. In order to separate these different influences, we first considered two classes of seesaw models, namely quasi-degenerate and strongly hierarchical right-handed neutrinos. As a generalisation, we presented a method that can be used to reconstruct the high energy seesaw parameters, among them the heavy right-handed neutrino masses, from low energy observables alone.}, subject = {Supersymmetrie}, language = {en} } @phdthesis{Kittel2004, author = {Kittel, Olaf}, title = {CP violation in production and decay of supersymmetric particles}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-12767}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2004}, abstract = {In dieser Dissertation untersuchen wir CP verletzende Effekte von MSSM-Phasen in Produktion und Zwei-Teilchen-Zerfaellen von Neutralinos, Charginos und Sfermionen. Fuer verschiedene supersymmetrische Prozesse definieren und berechnen wir CP-ungerade Asymmetrien, welche auf Spatprodukten basieren. Wir zeigen numerische Ergebnisse fuer Elektron-Positron-Kollisionen an einem zukuenftigen Linearbeschleuniger mit einer Energie von 500 - 800 GeV, hoher Luminositaet und longitudinal polarisierten Strahlen.}, subject = {Supersymmetrisches Teilchen}, language = {en} } @phdthesis{Redelbach2004, author = {Redelbach, Andreas}, title = {SUSY Seesaw model and phenomenological implications for leptonic processes at low energies and leptogenesis}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-10182}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2004}, abstract = {In this work the supersymmetric seesaw model and its effects on low-energy leptonic observables and thermal leptogenesis have been systematically investigated. Precision measurements will increase the sensitivity on lepton-flavor violating decays, particularly on Br(l_j->l_i gamma) and also on electric and magnetic dipole moments in the near future. In order to improve also the accuracy of theoretical predictions for these processes, we have performed a full one-loop calculation of the underlying supersymmetric processes taking into account the lepton masses. Since the mechanism of soft supersymmetry breaking (SSB) is completely unknown, a novel analysis beyond the often studied minimal Supergravity scenarios has been performed. This way it has been demonstrated that in the considered mSUGRA, AMSB, GMSB and gaugino mediated scenarios, the ongoing search for Br(mu->e gamma) can constrain fundamental SSB parameters and/or the seesaw parameters. On the other hand, the basic parameters of thermal leptogenesis, such as the CP asymmetry in the decays of the lightest right-handed Majorana neutrino, provide probes of the unknown complex orthogonal R-matrix of the seesaw model.}, subject = {Supersymmetrie}, language = {en} } @phdthesis{Dahnken2004, author = {Dahnken, Christopher}, title = {Spectral properties of strongly correlated electron systems}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-12238}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2004}, abstract = {We investigate the single particle static and dynamic properties at zero temperature within the Hubbard an three-band-Hubbard model for the superconducting copper oxides. Based on the recently proposed self-energy functional approach (SFA) [M.Potthoff, Eur. Phys. J. B 32 429 (2003)], we present an extension of the cluster-perturbation theory (CPT) to systems with spontaneous broken symmetry. Our method accounts for both short-range correlations and long-range order. Short-range correlations are accurately taken into account via the exact diagonalization of finite clusters. Long-range order is described by variational optimization of a ficticious symmetry-breaking field. In comparison with related cluster methods, our approach is more flexible and, for a given cluster size, less demanding numerically, especially at zero temperature. An application of the method to the antiferromagnetic phase of the Hubbard model at half-filling shows good agreement with results from quantum Monte-Carlo calculations. We demonstrate that the variational extension of the cluster-perturbation theory is crucial to reproduce salient features of the single-particle spectrum of the insulating cuprates. Comparison of the dispersion of the low-energy excitations with recent experimental results of angular resolved photoemission spectroscopy (ARPES) allows us to fix a consistent parameter set for the one-band Hubbard model with an additional hopping parameter t' along the lattice diagonal. The doping dependence of the single-particle excitations is studied within the t-t-U Hubbard model with special emphasis on the electron doped compounds. We show, that the ARPES results on the band structure and the Fermi surface of Nd{2-x}Ce_xCuOCl_{4-\delta} are naturally obtained within the t-t-U Hubbard model without further need for readjustment or fitting of parameters, as proposed in recent theoretical considerations. We present a theory for the photon energy and polarization dependence of ARPES intensities from the CuO2 plane in the framework of strong correlation models. The importance of surface states for the observed experimental facts is considered. We show that for electric field vector in the CuO_2 plane the 'radiation characteristics' of the O 2p_{\sigma} and Cu 3d_{x^2-y^2} orbitals are strongly peaked along the CuO_2 plane, i.e. most photoelectrons are emitted at grazing angles. This suggests that surface states play an important role in the observed ARPES spectra, consistent with recent data from Sr_2CuCl_2O_2. We show that a combination of surface state dispersion and Fano resonance between surface state and the continuum of LEED-states may produce a precipitous drop in the observed photoelectron current as a function of in-plane momentum, which may well mimic a Fermi-surface crossing. This effect may explain the simultaneous 'observation' of a hole-like and an electron-like Fermi surfaces in Bi_2Sr_2CaCu_2O_{8+\delta} at different photon energies.}, subject = {Hochtemperatursupraleiter}, language = {en} } @phdthesis{Volkmann2004, author = {Volkmann, Thorsten}, title = {Lattice gas models and simulations of epitaxial growth}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-13812}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2004}, abstract = {In this PhD thesis, we develop models for the numerical simulation of epitaxial crystal growth, as realized, e.g., in molecular beam epitaxy (MBE). The basic idea is to use a discrete lattice gas representation of the crystal structure, and to apply kinetic Monte Carlo (KMC) simulations for the description of the growth dynamics. The main advantage of the KMC approach is the possibility to account for atomistic details and at the same time cover MBE relevant time scales in the simulation. In chapter 1, we describe the principles of MBE, pointing out relevant physical processes and the influence of experimental control parameters. We discuss various methods used in the theoretical description of epitaxial growth. Subsequently, the underlying concepts of the KMC method and the lattice gas approach are presented. Important aspects concerning the design of a lattice gas model are considered, e.g. the solid-on-solid approximation or the choice of an appropriate lattice topology. A key element of any KMC simulation is the selection of allowed events and the evaluation of Arrhenius rates for thermally activated processes. We discuss simplifying schemes that are used to approximate the corresponding energy barriers if detailed knowledge about the barriers is not available. Finally, the efficient implementation of the MC kinetics using a rejection-free algorithm is described. In chapter 2, we present a solid-on-solid lattice gas model which aims at the description of II-VI(001) semiconductor surfaces like CdTe(001). The model accounts for the zincblende structure and the relevant surface reconstructions of Cd- and Te-terminated surfaces. Particles at the surface interact via anisotropic nearest and next nearest neighbor interactions, whereas interactions in the bulk are isotropic. The anisotropic surface interactions reflect known properties of CdTe(001) like the small energy difference between the c(2x2) and (2x1) vacancy structures of Cd-terminated surfaces. A key element of the model is the presence of additional Te atoms in a weakly bound Te* state, which is motivated by experimental observations of Te coverages exceeding one monolayer at low temperatures and high Te fluxes. The true mechanism of binding excess Te to the surface is still unclear. Here, we use a mean-field approach assuming a Te* reservoir with limited occupation. In chapter 3, we perform KMC simulations of atomic layer epitaxy (ALE) of CdTe(001). We study the self-regulation of the ALE growth rate and demonstrate how the interplay of the Te* reservoir occupation with the surface kinetics results in two different regimes: at high temperatures the growth rate is limited to one half layer of CdTe per ALE cycle, whereas at low enough temperatures each cycle adds a complete layer. The temperature where the transition between the two regimes occurs depends mainly on the particle fluxes. The temperature dependence of the growth rate and the flux dependence of the transition temperature are in good qualitative agreement with experimental results. Comparing the macroscopic activation energy for Te* desorption in our model with experimental values we find semiquantitative agreement. In chapter 4, we study the formation of nanostructures with alternating stripes during submonolayer heteroepitaxy of two different adsorbate species on a given substrate. We evaluate the influence of two mechanisms: kinetic segregation due to chemically induced diffusion barriers, and strain relaxation by alternating arrangement of the adsorbate species. KMC simulations of a simple cubic lattice gas with weak inter-species binding energy show that kinetic effects are sufficient to account for stripe formation during growth. The dependence of the stripe width on control parameters is investigated. We find an Arrhenius temperature dependence, in agreement with experimental investigations of phase separation in binary or ternary material systems. Canonical MC simulations show that the observed stripes are not stable under equilibrium conditions: the adsorbate species separate into very large domains. Off-lattice simulations which account for the lattice misfit of the involved particle species show that, under equilibrium conditions, the competition between binding and strain energy results in regular stripe patterns with a well-defined width depending on both misfit and binding energies. In KMC simulations, the stripe-formation and the experimentally reported ramification of adsorbate islands are reproduced. To clarify the origin of the island ramification, we investigate an enhanced lattice gas model whose parameters are fitted to match characteristic off-lattice diffusion barriers. The simulation results show that a satisfactory explanation of experimental observations within the lattice gas framework requires a detailed incorporation of long-range elastic interactions. In the appendix we discuss supplementary topics related to the lattice gas simulations in chapter 4.}, subject = {Kristallwachstum}, language = {en} } @phdthesis{Mueck2004, author = {M{\"u}ck, Alexander}, title = {The standard model in 5D : theoretical consistency and experimental constraints}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-10591}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2004}, abstract = {The four-dimensional Minkowski space is known to be a good description for space-time down to the length scales probed by the latest high-energy experiments. Nevertheless, there is the viable and exciting possibility that additional space-time structure will be observable in the next generation of collider experiments. Hence, we discuss different extensions of the standard model of particle physics with an extra dimension at the TeV-scale. We assume that some of the gauge and Higgs bosons propagate in one additional spatial dimension, while matter fields are confined to a four-dimensional subspace, the usual Minkowski space. After compactification on an S^1/Z_2 orbifold, an effective four-dimensional theory is obtained where towers of Kaluza-Klein (KK) modes, in addition to the standard model fields, reflect the higher-dimensional structure of space-time. The models are elaborated from the 5D Lagrangian to the Feynman rules of the KK modes. Special attention is paid to an appropriate generalization of the Rxi-gauge and the interplay between spontaneous symmetry breaking and compactification. Confronting the observables in 5D standard model extensions with combined precision measurements at the Z-boson pole and the latest data from LEP2, we constrain the possible size R of the extra dimension experimentally. A multi-parameter fit of all relevant input parameters leads to bounds for the compactification scale M=1/R in the range 4-6 TeV at the 2 sigma confidence level and shows how the mass of the Higgs boson is correlated with the size of an extra dimension. Considering a future linear e+e- collider, we outline the discovery potential for an extra dimension using the proposed TESLA specifications as an example. As a consistency check for the various models, we analyze Ward identities and the gauge boson equivalence theorem in W-pair production and find that gauge symmetry is preserved by a complex interplay of the Kaluza-Klein modes. In this context, we point out the close analogy between the traditional Higgs mechanism and mass generation for gauge bosons via compactification. Beyond the tree-level, the higher-dimensional models studied extensively in the literature and in the first part of this thesis have to be extended. We modify the models by the inclusion of brane kinetic terms which are required as counter terms. Again, we derive the corresponding 4D theory for the KK towers paying special attention to gauge fixing and spontaneous symmetry breaking. Finally, the phenomenological implications of the new brane kinetic terms are investigated in detail.}, subject = {Standardmodell }, language = {en} } @phdthesis{Nuernberger2004, author = {N{\"u}rnberger, Dieter}, title = {The Galactic Starburst Region NGC 3603 : exciting new insights on the formation of high mass stars}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-10440}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2004}, abstract = {One of the most fundamental, yet still unsolved problems in star formation research is addressed by the question "How do high mass stars form?". While most details related to the formation and early evolution of low mass stars are quite well understood today, the basic processes leading to the formation of high mass stars still remain a mystery. There is no doubt that low mass stars like our Sun form via accretion of gas and dust from their natal environment. With respect to the formation of high mass stars theorists currently discuss two possible scenarios controversely: First, similar to stars of lower masses, high mass stars form by continuous (time variable) accretion of large amounts of gas and dust through their circumstellar envelopes and/or disks. Second, high mass stars form by repeated collisions (coalescence) of protostars of lower masses. Both scenarios bear difficulties which impose strong constrains on the final mass of the young star. To find evidences for or against one of these two theoretical models is a challenging task for observers. First, sites of high mass star formation are much more distant than the nearby sites of low mass star formation. Second, high mass stars form and evolve much faster than low mass star. In particular, they contract to main sequence, hydrogen burning temperatures and densities on time scales which are much shorter than typical accretion time scales. Third, as a consequence of the previous point, young high mass stars are usually deeply embedded in their natal environment throughout their (short) pre-main sequence phase. Therefore, high mass protostars are rare, difficult to find and difficult to study. In my thesis I undertake a novel approach to search for and to characterize high mass protostars, by looking into a region where young high mass stars form in the violent neighbourhood of a cluster of early type main sequence stars. The presence of already evolved O type stars provides a wealth of energetic photons and powerful stellar winds which evaporate and disperse the surrounding interstellar medium, thus "lifting the courtains" around nearby young stars at a relatively early evolutionary stage. Such premises are given in the Galactic starburst region NGC 3603. Nevertheless, a large observational effort with different telescopes and instruments -- in particular, taking advantage of the high angular resolution and high sensitivity of near and mid IR instruments available at ESO -- was necessary to achieve the goals of my study. After a basic introduction on the topic of (high mass) star formation in Chapter 1, a short overview of the investigated region NGC 3603 and its importance for both galactic and extragalactic star formation studies is given in Chapter 2. Then, in Chapter 3, I report on a comprehensive investigation of the distribution and kinematics of the molecular gas and dust associated with the NGC 3603 region. In Chapter 4 I thoroughly address the radial extent of the NGC 3603 OB cluster and the spatial distribution of the cluster members. Together with deep Ks band imaging data, a detailed survey of NGC 3603 at mid IR wavelengths allows to search the neighbourhood of the cold molecular gas and dust for sources with intrinsic mid IR excess (Chapter 5). In Chapter 6 I characterize the most prominent sources of NGC 3603 IRS 9 and show that these sources are bona-fide candidates for high mass protostars. Finally, a concise summary as well as an outlook on future prospects in high mass star formation research is given in Chapter 7.}, subject = {Starburst-Galaxie}, language = {en} } @phdthesis{Siddiki2005, author = {Siddiki, Afif}, title = {Model calculations of current and density distributions in dissipative Hall bars}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-15100}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2005}, abstract = {In this work we examine within the self-consistent Thomas-Fermi-Poisson approach the low-temperature screening properties of a two-dimensional electron gas (2DEG) subjected to strong perpendicular magnetic fields. In chapter 3, numerical results for the unconfined 2DEG are compared with those for a simplified Hall-bar geometry realized by two different confinement models. It is shown that in the strongly nonlinear-screening limit of zero temperature the total variation of the screened potential is related by simple analytical expressions to the amplitude of an applied harmonic modulation potential and to the strength of the magnetic field. In chapter 4 we study the current and charge distribution in a two-dimensional electron system, under the conditions of the integer quantized Hall effect, on the basis of a quasilocal transport model, that includes nonlinear screening effects on the conductivity via the self-consistently calculated density profile. The existence of "incompressible strips" with integer Landau level filling factor is investigated within a Hartree-type approximation, and nonlocal effects on the conductivity along those strips are simulated by a suitable averaging procedure. This allows us to calculate the Hall and the longitudinal resistance as continuous functions of the magnetic field B, with plateaus of finite widths and the well-known, exactly quantized values. We emphasize the close relation between these plateaus and the existence of incompressible strips, and we show that for B values within these plateaus the potential variation across the Hall bar is very different from that for B values between adjacent plateaus, in agreement with recent experiments. We have improved on the previous chapter by a critical investigation of the impurity potential profiles and obtained reasonable estimates of the range and the amplitude of the potential fluctuations. We added a harmonic perturbation potential to the confining potential in order to generate the long-range-part of the overall impurity potential in the translation invariant model. This treatment of the long-range fluctuations allowed us to resolve apparent discrepancies such as the dependence of the QH plateau width on the mobility and to understand the crossing values of the high and low temperature Hall resistances. An interesting outcome of this model is that, it predicts different crossing values depending on the sample width and mobility. In chapter 6 we brie y report on theoretical and experimental investigations of a novel hysteresis effect that has been observed on the magneto-resistance (MR) of quantum-Hall (QH) bilayer systems in magnetic field (B) intervals, in which one layer is in a QH-plateau while the other is near an edge of a QH-plateau. We extend a recent approach to the QH effect, based on the Thomas-Fermi-Poisson theory and a local conductivity model to the bilayer system. This approach yields very different density and potential landscapes for the B-values at different edges of a QH plateau. Combining this with the knowledge about extremely long relaxation times to the thermodynamic equilibrium within the plateau regime, we simulate the hysteresis in the "active" current-carrying layer by freezing-in the electron density in the other, "passive", layer at the profile corresponding to the low-B edge of its QH plateau as B is swept up, and to the profile at the high-B edge as B is swept down. The calculated MR hysteresis is in good qualitative agreement with the experiment. If we use the equilibrium density profile, we obtain excellent agreement with an "equilibrium" measurement, in which the system was heated up to ~ 10K and cooled down again at each sweep step.}, subject = {Elektronengas}, language = {en} } @phdthesis{Pahlen2005, author = {Pahlen, Federico von der}, title = {Polarization and Spin Effects in Production and Decay of Charginos and Neutralinos at a Muon Collider}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-18421}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2005}, abstract = {The mechanism of spontaneous symmetry breaking is essential to provide masses to the W and Z gauge bosons and fermions of the SM. We hope to elucidate this mechanism at the next generation of colliders. While the SM has been tested with astonishing precision it is believed to be an effective theory of a more fundamental Great Unified Theory. SUSY is one of the most attractive extensions of the SM of particle physics. Therefore, the search for SUSY is a top priority at the next generation of colliders. Once Higgs bosons are discovered, a precise determination of their properties is necessary to differentiate between different models, in particular the MSSM. A muon collider, running at center of mass energies around the neutral Higgs boson resonances, would allow precise measurements of masses and widths, as well as the couplings to their decay products. In particular their couplings to supersymmetric particles are essential to probe SUSY. Therefore, we study the decays of the heavier CP-even and CP-odd Higgs bosons into lighter chargino or neutralino pairs. In this thesis we have analyzed the polarization effects of the beams and the charginos and neutralinos produced in mu+ mu- annihilation around the center of mass energies of the Higgs boson resonances H and A. For the production of equal charginos we have shown that the ratio of H-chargino and A-chargino couplings can be precisely determined independently of the chargino decay mechanism. This method avoids reference to other experiments and makes only a few model-dependent assumptions. Here we have analyzed the effect of the energy spread and of the error from the non-resonant channels, including an irreducible standard model background contribution. For small tan(beta) the process yields large cross sections of up to a pb. For the production of two different charginos we have shown that the H-A interference can be analyzed using asymmetries of the charge conjugated processes. The asymmetries depend on the muon longitudinal beam polarizations and vanish for unpolarized beams. For the chargino pair production with subsequent two-body decay of one of the charginos we have shown that charge and beam polarization asymmetries in the energy distributions of the decay particles are sensitive to the interference of scalar exchange channels with different CP quantum numbers. This process provides unique information on the interference of overlapping Higgs boson resonances. The effect is larger for regions of parameter space with intermediate values of tan(beta) and light sleptons or LSP neutralinos. For the chargino pair production with subsequent two-body decays of both charginos we have defined energy distribution and angular asymmetries in the final particles, in order to analyze the spin-spin correlations of the charginos. The transverse polarizations of the charginos are sensitive to the CP quantum number of the exchanged Higgs bosons and can thus be used to separate overlapping resonances, as well as to determine the CP quantum number of a single resonance. For equal charginos, these asymmetries are not sensitive to the interference of CP-even and CP-odd Higgs exchange channels. For the neutralino pair production in mu+ mu- annihilation we study similar processes as for chargino production. Line shape measurements of neutralino pair production allow to precisely determine the ratio of H-neutralino and A-neutralino couplings. Neutralino pair production with subsequent two-body decay of one of the neutralinos in the intermediate tan(beta) region is sensitive to the interference of H and A and may be measured with a large statistical significance. The Majorana nature of the neutralinos implies that the beam polarization asymmetries vanish for the remaining production channels. For neutralino pair production with subsequent two-body decays of both neutralinos we analyze similar observables as in chargino production. The main difference consists in the intrinsic relative CP quantum number of the neutralino pair, which depends on the chosen scenario. We have thus shown that the interaction of the Higgs bosons to the gaugino-higgsino sector can be probed at a muon collider in chargino and neutralino pair production, both analyzing the production line-shape around the resonances as well as studying the chargino and neutralino polarizations via their decays.}, subject = {Neutralino}, language = {en} } @phdthesis{Joestingmeier2005, author = {J{\"o}stingmeier, Martin}, title = {On the competition of superconductivity, antiferromagnetism and charge order in the high-Tc compounds}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-13036}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2005}, abstract = {Diese Arbeit l{\"a}ßt sich in zwei grobe Abschnitte gliedern. Der erste Teil umfaßt die Kapitel 1-3, in denen drei verschiedene Konzepte beschrieben werden, die zum Verst{\"a}ndis stark korrelierter Vielteilchen-Systeme dienen. Dies sind zun{\"a}chst einmal die SO(5)-Theorie in Kapitel 3, die den allgemeinen Rahmen vorgibt und auf der numerischen Seite die Stochastische Reihen Entwicklung (SSE) in Kapitel 1 und der Contractor Renormierungsgruppen Ansatz (CORE), s.Kapitel 2). Die zentrale Idee dieser Dissertationsschrift besteht darin, diese verschiedenen Konzepte zu kombinieren, um ein besseres Verst{\"a}ndnis der Hochtemperatursupraleiter zu erhalten. Im zweiten Teil dieser Arbeit (Kap. 4 und Kap. 5) werden die so gewonnenen Ergebnisse dargestellt. Die zentrale Idee dieser Arbeit, d.h. die Kombination der SO(5)-Theorie mit den F{\"a}higkeiten bosonischer Quanten-Monte-Carlo Verfahren und den {\"u}berlegungen der Renormierungsgruppe, hat sich sich am Beispiel der Physik der Hochtemperatur-Supraleiter als sehr tragf{\"a}hig erwiesen. Die numerischen Simulationen reproduzieren bei den behandelten Modelle eine Reihe wichtiger experimenteller Daten. Die Grundlage f{\"u}r eine k{\"u}nftige weitere schrittweise Erweiterung des Modells wurde so geschaffen. Eine offene Frage ist z.B. die Restaurierung der SO(5)-Symmetrie an einem multi-kritischen Punkt, wenn die l{\"a}ngerreichweitigen Wechselwirkungen mit in das Modell einbezogen sind.}, subject = {Hochtemperatursupraleitung}, language = {en} } @phdthesis{Brunner2005, author = {Brunner, Raimund}, title = {Analyse optischer Heterodynsignale zur dynamischen Charakterisierung von Diodenlasern}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-17195}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2005}, abstract = {Die stetige Degradation von Halbleiterlasern, speziell bei Bleichalkogenidlasern, erfordert in spektroskopischen Systemen eine regelm{\"a}ßige {\"U}berwachung typischer Eigenschaften wie Abstimmcharakteristik und Linienbreite. Im Hinblick auf einen m{\"o}glichst hohen Automatisierungsgrad wird langfristig eine Online-Analysemethode zur {\"U}berwachung notwendig sein. Die {\"u}blicherweise verwendete Methode, den Laserarbeitspunkt {\"u}ber zugrunde liegende Modenkarten einzustellen, hat den gravierenden Nachteil, dass solche Modenkarten in der Regel nicht unter dynamischen Modulationsbedingungen vermessen wurden. Gerade im dynamischen Fall sind diese Karten empfindlich abh{\"a}ngig gegen{\"u}ber Ver{\"a}nderungen durch Zyklieren und Degradieren des Lasers. Etalons (Etalonsignale) sind bez{\"u}glich der Abstimmcharakteristik nicht zuverl{\"a}ssig genug und von daher f{\"u}r eine w{\"u}nschenswerte Automatisierung nicht ausreichen. Modenspr{\"u}nge oder schwache R{\"u}ckkopplungseffekte lassen sich im Interferogramm nicht ohne weiteres identifiziert. Eine erweiterte Analyse der St{\"o}rungen dieser Interferogramme im Zeit-Frequenzbereich mittels einer AOK(Adaptive Optimal Kernel)-Transformation erwies sich speziell bei Signalen mit wenigen Perioden als deutlich aussagekr{\"a}ftiger. Mittels optischer Homodynmischung wurde die Linienbreite von Bleichalkogenidlasern ermittelt. Bei inkoh{\"a}renter {\"U}berlagerung entspricht die spektrale Verteilung der Mischung der Faltung der urspr{\"u}nglichen Verteilung mit sich selbst. Der Laser wird dabei nicht abgestimmt, die optische Laufzeitverz{\"o}gerung wurde mittels integrierter White-Zelle realisiert. Es wurde beobachtet, dass je nach Grad des Rauschens des Injektionsstroms, das Linienbreitenprofil von Lorentz nach Gauß {\"u}berging. Mit einem externen CO2-Laser als lokalen Oszillator wurden Heterodynmessungen durchgef{\"u}hrt. Die Linienbreite eines CO2-Lasers ist mit wenigen kHz im Vergleich zu derjenigen eines Bleichalkogenidlasers vernachl{\"a}ssigbar und die {\"U}berlagerung erfolgt absolut inkoh{\"a}rent. Gemessen wurden spektrale Verteilungen mit typischem Lorentzprofil von 10 MHz bis zu 100 MHz und dar{\"u}ber hinaus. Auff{\"a}llig waren h{\"a}ufig symmetrische Nebenpeaks, die in den Bereichen der Seitenflanken des Lorentzprofils auftraten. Anhand einer numerischen Simulation eines Modells einer Laserdiode, basierend auf Ratengleichungen mit f{\"u}r Bleichalkogenidlasern typischen Parameterwerten, konnte verdeutlicht werden, dass sich durch das nichtlineare Lasermodell ausgepr{\"a}gte Vielfache von Resonanzen bereits im Abstand von 25 MHz ausbilden k{\"o}nnen. Derartige Resonanzen tauchen im E-Feld-Spektrum als typische Relaxationsoszillationen in den Seitenb{\"a}ndern wieder auf und erkl{\"a}ren die in der Messung beobachteten Nebenpeaks innerhalb der spektralen Verteilung. Die St{\"a}rke der Seitenb{\"a}nder ist ein Maß f{\"u}r die Korrelation zwischen Phasen- und Amplitudenfluktuationen. Das Modell f{\"u}r die numerische Berechnung des E-Feldes wurde mit einem thermischen Verhalten erweitert. Eine umfassende Charakterisierungsmethode zur automatisierten Einstellung eines modulierten Lasersystems muss dynamisch und zeitaufgel{\"o}st erfolgen. Die Auswertung optischer Mischfrequenzen beschr{\"a}nkt sich dabei nicht mehr auf die direkte Interpretation von einzelnen Spektren, sondern erweitert sich auf die Analyse im Zeit-Frequenzraum. F{\"u}r eine direkte und schnelle Zeitfrequenztransformation bietet sich ein „Gefensterte Fouriertransformation" (STFT) an, die sich außerdem relativ einfach in moderne Signalprozessortechnik implementieren l{\"a}sst. Sie erweist sich als sehr robust und f{\"u}r die hier erforderliche Analyse von Heterodynsignalen als ausreichend. Mit der Festlegung des Analysefensters innerhalb einer STFT ist die Aufl{\"o}sung in Zeit und Frequenz fest definiert. Analysen von Mischsignalen mit einer kontinuierlichen Wavelettransformation haben vergleichsweise gezeigt, dass Details im Zeitfrequenzraum zwar besser herausgearbeitet werden k{\"o}nnen, jedoch ist der Rechenaufwand durch die variable Skalierung und somit stark redundante Analyse und ihre Darstellung unverh{\"a}ltnism{\"a}ßig gr{\"o}ßer. Eine Analyse des Linienbreitenprofils erfolgt dabei {\"u}ber die Entwicklung der Skalierung eines Signals. Die {\"u}ber Heterodynsignale ermittelte effektive Linienbreite bei einer modulierten Abstimmung sollte eher als „dynamische" oder „intrinsische" Laserlinienbreite bezeichnet werden. Eine direkte Korrelation der Frequenzvariation des Lasers mit dem Stromrauschen des Injektionsstroms ist offensichtlich. Die wirksame Bandbreite des Stromrauschens wird durch die Systemelektronik einerseits und die Modulationsbandbreite des Lasers andererseits begrenzt. Außer den wichtigen Parametern wie Abstimmung und Linienbreite lassen sich {\"u}ber die dynamische Zeitfrequenzanalyse von Heterodynsignalen dar{\"u}ber hinaus weitere Ph{\"a}nomene wie R{\"u}ckkopplung, Moden{\"u}berlagerung oder Einschwingverhalten aufgrund direkter Kopplung zwischen Intensit{\"a}ts­ und Frequenzmodulation beobachten.}, subject = {Laserdiode}, language = {de} } @phdthesis{Csallner2006, author = {Csallner, Sigrun}, title = {Produktion und Nachweis schwerer Selektronen}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-22433}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2006}, abstract = {Wir studieren die Produktion und den Nachweis von Selektronen mit Massen jenseits der Schwelle zur Paarerzeugung an k{\"u}nftigen Linearbeschleunigern mit Schwerpunktsenergien von 500 GeV und 800 GeV. Hierzu betrachten wir die Produktion von linken und rechten Selektronen in Assoziation mit dem jeweils leichtesten Neutralino oder Chargino durch Elektron-Elektron-, Elektron-Positron- und Elektron-Photon-Streuung im Rahmen des MSSM. Die Produktion durch Elektron-Elektron-Streuung untersuchen wir zus{\"a}tzlich in zwei erweiterten Modellen, dem NMSSM und einem E6-Modell mit einem zus{\"a}tzlichen U(1)-Eichfaktor.}, subject = {Linearbeschleuniger}, language = {de} } @phdthesis{Bretz2006, author = {Bretz, Thomas}, title = {Observations of the Active Galactic Nucleus 1ES1218+304 with the MAGIC-telescope}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-19240}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2006}, abstract = {The astronomical exploration at energies between 30\,GeV and \$\lesssim\$\,350\,GeV was the main motivation for building the \MAGIC-telescope. With its 17\,m \diameter\ mirror it is the worldwide largest imaging air-Cherenkov telescope. It is located at the Roque de los Muchachos at the Canary island of San Miguel de La Palma at 28.8\$^\circ\$\,N, 17.8\$^\circ\$\,W, 2200\,m a.s.l. The telescope detects Cherenkov light produced by relativistic electrons and positrons in air showers initiated by cosmic gamma-rays. The imaging technique is used to powerfully reject the background due to hadronically induced air showers from cosmic rays. Their inverse power-law energy-distribution leads to an increase of the event rate with decreasing energy threshold. For \MAGIC this implies a trigger rate in the order of 250\,Hz, and a correspondingly large data stream to be recorded and analyzed. A robust analysis software package, including the general framework \MARS, was developed and commissioned to allow automation, necessary for data taken under variable observing conditions. Since many of the astronomical sources of high-energy radiation, in particular the enigmatic gamma-ray bursts, are of a transient nature, the telescope was designed to allow repositioning in several tens of seconds, keeping a tracking accuracy of \$\lesssim\,\$0.01\$^\circ\$. Employing a starguider, a tracking accuracy of \$\lesssim\,\$1.3\,minutes of arc was obtained. The main class of sources at very high gamma-ray energies, known from previous imaging air-Cherenkov telescopes, are Active Galactic Nuclei with relativistic jets, the so-called high-peaked Blazars. Their spectrum is entirely dominated by non-thermal emission, spanning more than 15 orders of magnitude in energy, from radio to gamma-ray energies. Predictions based on radiation models invoking a synchrotron self-Compton or hadronic origin of the gamma-rays suggest, that a fairly large number of them should be detectable by \MAGIC. Promising candidates have been chosen from existing compilations, requiring high (synchrotron) X-ray flux, assumed to be related to a high (possibly inverse-Compton) flux at GeV energies, and a low distance, in oder to avoid strong attenuation due to pair-production in interactions with low-energy photons from the extragalactic background radiation along the line of sight. Based on this selection the first \AGN, emitting gamma-rays at 100\,GeV, 1ES\,1218+304 at a redshift of \$z=0.182\$, was discovered, one of the two farthest known \AGN emitting in the TeV energy region. In this context, the automated analysis chain was successfully demonstrated. The source was observed in January 2005 during six moonless nights for 8.2\,h. At the same time the collaborating \KVA-telescope, located near the \MAGIC site, observed in the optical band. The lightcurve calculated showed no day-to-day variability and is compatible with a constant flux of \$F(\$\,\$>\$\,\$100\,\mbox{GeV})=(8.7\pm1.4) \cdot 10^{-7}\,\mbox{m}^{-2}\,\mbox{s}^{-1}\$ within the statistical errors. A differential spectrum between 87\,GeV and 630\,GeV was calculated and is compatible with a power law of \$F_E(E) = (8.1\pm 2.1) \cdot 10^{-7}(E/\mbox{250\,GeV})^{-3.0\pm0.4}\,\mbox{TeV}^{-1}\,\mbox{m}^{-2}\,\mbox{s}^{-1}\$ within the statistical errors. The spectrum emitted by the source was obtained by taking into account the attenuation due to pair-production with photons of the extragalactic background at low photon energies. A homogeneous, one-zone synchrotron self-Compton model has been fitted to the collected multi-wavelength data. Using the simultaneous optical data, a best fit model could be obtained from which some physical properties of the emitting plasma could be inferred. The result was compared with the so-called {\em Blazar sequence}.}, subject = {Aktiver galaktischer Kern}, language = {en} } @phdthesis{Ruttor2006, author = {Ruttor, Andreas}, title = {Neural Synchronization and Cryptography}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-23618}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2006}, abstract = {Neural networks can synchronize by learning from each other. For that purpose they receive common inputs and exchange their outputs. Adjusting discrete weights according to a suitable learning rule then leads to full synchronization in a finite number of steps. It is also possible to train additional neural networks by using the inputs and outputs generated during this process as examples. Several algorithms for both tasks are presented and analyzed. In the case of Tree Parity Machines the dynamics of both processes is driven by attractive and repulsive stochastic forces. Thus it can be described well by models based on random walks, which represent either the weights themselves or order parameters of their distribution. However, synchronization is much faster than learning. This effect is caused by different frequencies of attractive and repulsive steps, as only neural networks interacting with each other are able to skip unsuitable inputs. Scaling laws for the number of steps needed for full synchronization and successful learning are derived using analytical models. They indicate that the difference between both processes can be controlled by changing the synaptic depth. In the case of bidirectional interaction the synchronization time increases proportional to the square of this parameter, but it grows exponentially, if information is transmitted in one direction only. Because of this effect neural synchronization can be used to construct a cryptographic key-exchange protocol. Here the partners benefit from mutual interaction, so that a passive attacker is usually unable to learn the generated key in time. The success probabilities of different attack methods are determined by numerical simulations and scaling laws are derived from the data. If the synaptic depth is increased, the complexity of a successful attack grows exponentially, but there is only a polynomial increase of the effort needed to generate a key. Therefore the partners can reach any desired level of security by choosing suitable parameters. In addition, the entropy of the weight distribution is used to determine the effective number of keys, which are generated in different runs of the key-exchange protocol using the same sequence of input vectors. If the common random inputs are replaced with queries, synchronization is possible, too. However, the partners have more control over the difficulty of the key exchange and the attacks. Therefore they can improve the security without increasing the average synchronization time.}, language = {en} } @phdthesis{Pfannes2006, author = {Pfannes, Jan M. M.}, title = {Explosions of Rotating White Dwarfs}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-20872}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2006}, abstract = {The impact of rapid rotation of the supernova progenitor star (white dwarf) on its explosion (type Ia supernova) is investigated. Different explosion mechanisms are employed.}, subject = {Weißer Zwerg}, language = {en} } @phdthesis{Bruenger2007, author = {Br{\"u}nger, Christian}, title = {Numerical Studies of Quantum Spin Systems}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-26439}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2007}, abstract = {Der erste Teil der Arbeit widmet sich der Untersuchung des Bilayer-Heisenberg-Modells und des zweidimensionalen Kondo-Necklace-Modells. Beide Modelle weisen einen Quantenphasen{\"u}bergang zwischen einer geordneten und einer ungeordneten Phase auf. In dieser Arbeit richtet sich das Interesse insbesondere auf die Kopplung der kritischen Fluktuationen an ein in das System eingebundenes Loch. Mittels eines selbstkonsistenten Born'schen N{\"a}herungsverfahrens wird gezeigt, dass das Loch mit den Magnonen derart wechselwirkt, dass dessen Quasiteilchengewicht am quantenkritischen Punkt verschwindet. Um diesen Aspekt weiter zu untersuchen, wird das Verhalten des Quasiteilchengewichts im Bereich der kritischen Kopplung auch mit Quanten-Monte-Carlo-Methoden analysiert. Desweiteren werden die dynamischen Eigenschaften des Loches im magnetischen Hintergrund untersucht. Im zweiten Teil dieser Arbeit gilt das Interesse der Untersuchung des Spiral-Staircase-Heisenberg-Modells. Dieses besteht aus zwei, zu einer Spinleiter ferromagnetisch gekopplten Spin-1/2-Ketten, wobei die antiferromagnetische Kopplung innerhalb der zweiten Kette durch Windung der Leiter variiert werden kann. Dieses Model eignet sich, den {\"U}bergang zwischen einer Spin-1/2-Kette ohne Spinl{\"u}cke und einer Spin-1-Kette mit Spinl{\"u}cke zu studieren. Besondere Beachtung ist dem {\"O}ffnen der Spinl{\"u}cke in Abh{\"a}ngigkeit der ferromagnetischen Kopplung zwischen den Leiterbeinen geboten. Es stellt sich heraus, dass das System, abh{\"a}ngig von der Leiterwindung, wesentliche Unterschiede im Skalierungsverhalten der Spinl{\"u}cke aufweist. Desweiteren wird mittels der String-Order-Parameter gezeigt, dass das Spiral-Staircase-Heisenberg-Modell trotz des unterschiedlichen Skalierungsverhaltens der Spinl{\"u}cke und unabh{\"a}ngig von der Wahl der Parameter sich stets in der Haldane-Phase befindet. Die Analyse der Modelle bedient sich haupts{\"a}chlich Quanten-Monte-Carlo-Methoden, aber auch exakter Diagonalisierungstechniken, sowie auf Molekularfeldn{\"a}herungen gest{\"u}tzten Rechnungen.}, subject = {Spinsystem}, language = {en} } @phdthesis{Zeiner2007, author = {Zeiner, J{\"o}rg}, title = {Noncommutative Quantumelectrodynamics from Seiberg-Witten Maps to All Orders in Theta}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-23363}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2007}, abstract = {The basic question which drove our whole work was to find a meaningful noncommutative gauge theory even for the time-like case (\$\theta^{0 i} \neq 0\$). In order to be able to tackle questions regarding unitarity, it is not sufficient to consider theories which include the noncommutative parameter only up to a finite order. The reason is that in order to investigate tree-level unitarity or the optical theorem in loops one has to know the behavior of the noncommutative theory for center-of-mass energies much greater than the noncommutative scale. Therefore an effective theory, that is by construction only valid up to the noncommutative scale, isn't sufficient for our purpose. Our model is based on two fundamental assumptions. The first assumption is given by the commutation relations \eqref{eq:ncalg}. This led to the Moyal-Weyl star-product \eqref{eq:astproduct2} which replaces all point-like products between two fields. The second assumption is to assume that the model built this way is not only invariant under the noncommutative gauge transformation but also under the commutative one. In order to obtain an action of such a model one has to replace the fields by their appropriate \swms. We chose the gauge fixed action \eqref{eq:actioncgf} as the fundamental action of our model. After having constructed the action of the NCQED including the {\swms} we were confronted with the problem of calculating the {\swms} to all orders in \$\tMN\$. By means of \cite{bbg} we could calculate the {\swms} order by order in the gauge field, where each order in the gauge field contains all orders in the noncommutative parameter (\cf chapter \ref{chapter:swms}). By comparing the maps with the result we obtained from an alternative ansatz \cite{bcpvz}, we realized that already the simplest {\swm} for the gauge field is not unique. In chapter \ref{chapter:ambiguities} we examined this ambiguity, which we could parametrised by an arbitrary function \$\astf\$. The next step was to derive the Feynman rules for our NCQED. One finds that the propagators remain unchanged so that the free theory is equal to the commutative QED. The fermion-fermion-photon vertex contains not only a phase factor coming from the Moyal-Weyl star-product but also two additional terms which have their origin in the \swms. Beside the 3-photon vertex which is already present in NCQED without {\swms} and which has also additional terms coming from the \swms, too, one has a contact vertex which couples two fermions with two photons. After having derived all the vertices we calculated the pair annihilation scattering process \$e^+ e^- \rightarrow \gamma \gamma\$ at Born level. By choosing the parameter \$\kggg = 1\$ (\cf section \ref{sec:represent}), we found that the amplitude of the pair annihilation process becomes equal to the amplitude of the NCQED without \swms. This means that, at least for this process, the NCQED excluding {\swms} is only a special case of NCQED including \swms. On the basis of the pair annihilation process, we afterwards investigated tree-level unitarity. In order to satisfy the tree-level unitarity we had to constrain the arbitrary function \$\astf\$. We found that the series expansion of \$\astf\$ has to start with unity. In addition, the even part of the function must not increase faster than \$s^{-1/2} \log(s)\$ for \$s \rightarrow \infty\$, whereas the odd part of the \$\astf\$-function can't be constrained, at least by the process we considered. By assuming these constrains for the \$\astf\$-function, we could show that tree-level unitarity is satisfied if one incorporates the uncertainties present in the energy and the momenta of the scattered particles, \ie the uncertainties of the center-of-mass energy and the scattering angles. This uncertainties are not exclusively present due to the finite experimental resolution. A delta-like center-of-mass energy as well as delta-like momenta are in general not possible because the scattered particles are never exact plane waves.}, subject = {Raum-Zeit}, language = {en} } @phdthesis{Alboteanu2007, author = {Alboteanu, Ana Maria}, title = {The Noncommutative Standard Model : Construction Beyond Leading Order in Theta and Collider Phenomenology}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-24334}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2007}, abstract = {Trotz seiner pr{\"a}zisen {\"U}bereinstimmung mit dem Experiment ist die G{\"u}ltigkeit des Standardmodells (SM) der Elementarteilchenphysik bislang nur bis zu einer Energieskala von einigen hundert GeV gesichert. Abgesehen davon erweist sich schon das Einbinden der Gravitation in einer einheitlichen Beschreibung aller fundamentalen Wechselwirkungen als ein durch gew{\"o}hnliche Quantenfeldtheorie nicht zu l{\"o}sendes Problem. Das Interesse an Quantenfeldtheorien auf einer nichtkommutativen Raumzeit wurde durch deren Vorhersage als niederenergetischer Limes von Stringtheorien erweckt. Unabh{\"a}ngig davon, kann die Nichtlokalit{\"a}t einer solchen Theorie den Rahmen zur Einbeziehung der Gravitation in eine vereinheitlichende Theorie liefern. Die Hoffnung besteht, dass die Energieskala Lambda_NC, ab der solche Effekte sichtbar werden k{\"o}nnen und f{\"u}r die es einerlei theoretischen Vorhersagen gibt, schon bei der n{\"a}chsten Generation von Beschleunigern erreicht wird. Auf dieser Annahme beruht auch die vorliegende Arbeit, im Rahmen deren eine m{\"o}gliche Realisierung von Quantenfeldtheorien auf nichtkommutativer Raumzeit auf ihre ph{\"a}nomenologischen Konsequenzen hin untersucht wurde. Diese Arbeit ist durch fehlende LHC (Large Hadron Collider) Studien f{\"u}r nichkommutative Quantenfeldtheorien motiviert. Im ersten Teil des Vorhabens wurde der hadronische Prozess pp-> Z gamma -> l+l- gamma am LHC sowie die Elektron-Positron Paarvernichtung in ein Z-Boson und ein Photon am ILC (International Linear Collider) auf nichtkommutative Signale hin untersucht. Die ph{\"a}nomenlogischen Untersuchungen wurden im Rahmen dieses Modells in erster Ordnung des nichtkommutativen Parameters Theta durchgef{\"u}hrt. Eine nichtkommutative Raumzeit f{\"u}hrt zur Brechung der Rotationsinvarianz bez{\"u}glich der Strahlrichtung der einlaufenden Teilchen. Im differentiellen Wirkungsquerschnitt f{\"u}r Streuprozesse {\"a}ussert sich dieses als eine azimuthale Abh{\"a}ngigkeit, die weder im SM noch in anderen Modellen jenseits des SM auftritt. Diese klare, f\"ur nichtkommutative Theorien typische Signatur kann benutzt werden, um nichtkommutative Modelle von anderen Modellen, die neue Physik beschreiben, zu unterscheiden. Auch hat es sich erwiesen, dass die azimuthale Abh{\"a}ngigkeit des Wirkungsquerschnittes am besten daf\"ur geeignet ist, um die Sensitivit{\"a}t des LHC und des ILC auf der nichtkommutativen Skala \$\Lnc\$ zu bestimmen. Im ph{\"a}nomenologischen Teil der Arbeit wurde herausgefunden, dass Messungen am LHC f{\"u}r den Prozess pp-> Z gamma-> l+l- gamma nur in bestimmten F{\"a}llen auf nichtkommutative Effekte sensitiv sind. F{\"u}r diese F{\"a}lle wurde f{\"u}r die nichtkommutative Energieskala Lambda_NC eine Grenze von Lambda_NC > 1.2 TeV bestimmt. Diese ist um eine Gr{\"o}ßenordnung h{\"o}her als die Grenzen, die von bisherigen Beschleunigerexperimenten hergeleitet wurden. Bei einem zuk{\"u}nftigen Linearbeschleuniger, dem ILC, wird die Grenze auf Lambda_NC im Prozess e^+e^- -> Z gamma -> l^+ l^- gamma wesentlich erh{\"o}ht (bis zu 6 TeV). Abgesehen davon ist dem ILC gerade der f{\"u}r den LHC kaum zug{\"a}ngliche Parameterbereich der nichtkommutativen Theorie erschlossen, was die Komplementarit{\"a}t der beiden Beschleunigerexperimente hinsichtlich der nichtkommutativen Parameter zeigt. Der zweite Teil der Arbeit entwickelte sich aus der Notwendigkeit heraus, den G{\"u}ltigkeitsbereich der Theorie zu h{\"o}heren Energien hin zu erweitern. Daf{\"u}r haben wir den neutralen Sektor des nichtkommutativen SM um die n{\"a}chste Ordnung in Theta erg{\"a}nzt. Es stellte sich wider Erwarten heraus, dass die Theorie dabei um einige freie Parameter erweitert werden muss. Die zus{\"a}tzlichen Parameter sind durch die homogenen L{\"o}sungen der Eich{\"a}quivalenzbedingungen gegeben, welche Ambiguit\"aten der Seiberg-Witten Abbildungen darstellen. Die allgemeine Erwartung war, dass die Ambiguit{\"a}ten Feldredefinitionen entsprechen und daher in den Streumatrixelementen verschwinden m\"ussen. In dieser Arbeit wurde jedoch gezeigt, dass dies ab der zweiten Ordnung in Theta nicht der Fall ist und dass die Nichteindeutigkeit der Seiberg-Witten Abbildungen sich durchaus in Observablen niederschl{\"a}gt. Die Vermutung besteht, dass jede neue Ordnung in Theta neue Parameter in die Theorie einf{\"u}hrt. Wie weit und in welche Richtung die Theorie auf nichtkommutativer Raumzeit entwickelt werden muss, kann jedoch nur das Experiment entscheiden.}, subject = {Feldtheorie}, language = {en} } @phdthesis{Karg2007, author = {Karg, Stefan}, title = {Calculations of multi-particle processes at the one-loop level: precise predictions for the LHC}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-27505}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2007}, abstract = {The Standard Model (SM) of elementary particle physics provides a uniform framework for the description of three fundamental forces, the electromagnetic and weak forces, describing interactions between quarks and leptons, and the strong force, describing a much stronger interaction between the coloured quarks. Numerous experimental tests have been performed in the last thirty years, showing a spectacular agreement with the theoretical predictions of the Standard Model, even at the per mille level, therefore validating the model at the quantum level. An important cornerstone of the Standard Model is the Higgs mechanism, which provides a possible explanation of electroweak symmetry breaking, responsible for the masses of elementary fermions and the W and Z bosons, the carriers of the weak force. This mechanism predicts a scalar boson, the Higgs boson, which has escaped its discovery so far. If the Higgs mechanism is indeed realised in nature, the upcoming Large Hadron Collider (LHC) at CERN will be able to find the associated Higgs boson. The discovery of a Higgs boson by itself is not sufficient to establish the Higgs mechanism, the basic ingredient being the Higgs potential which predicts trilinear and quartic couplings. These have to be confirmed experimentally by the study of multi-Higgs production. We therefore present a calculation of the loop-induced processes gg to HH and gg to HHH, and investigate the observability of multi-Higgs boson production at the LHC in the Standard Model and beyond. While the SM cross sections are too small to allow observation at the LHC, we demonstrate that physics beyond the SM can lead to amplified, observable cross sections. Furthermore, the applicability of the heavy top quark approximation in two- and three-Higgs boson production is investigated. We conclude that multi-Higgs boson production at the SuperLHC is an interesting probe of Higgs sectors beyond the SM and warrants further study. Despite the great success of the SM, it is widely believed that this model cannot be valid for arbitrarily high energies. The LHC will probe the TeV scale and theoretical arguments indicate the appearance of physics beyond the SM at this scale. The search for new physics requires a precise understanding of the SM. Precise theoretical predictions are needed which match the accuracy of the experiments. For the LHC, most analyses require next-to-leading order (NLO) precision. Only then will we be able to reliably verify or falsify different models. At the LHC, many interesting signatures involve more than two particles in the final state. Precise theoretical predictions for such multi-leg processes are a highly nontrivial task and new efficient methods have to be applied. The calculation of the process PP to VV+jet at NLO is an important background process to Higgs production in association with a jet at the LHC. We compute the virtual corrections to this process which form the "bottleneck" for obtaining a complete NLO prediction. The resulting analytic expressions are generated with highly automated computer routines and translated into a flexible Fortran code, which can be employed in the computation of differential cross sections of phenomenological interest. The obtained results for the virtual corrections indicate that the QCD corrections are sizable and should be taken into account in experimental studies for the LHC.}, subject = {Higgs-Teilchen}, language = {en} } @phdthesis{Schmidt2008, author = {Schmidt, Manuel J.}, title = {Replica Symmetry Breaking at Low Temperatures}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-30660}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2008}, abstract = {In this thesis, the low-temperature regime of replica symmetry breaking in the SK-model has been thoroughly investigated. In order to access this regime and to perform self-consistence calculations with high accuracy at high orders of replica symmetry breaking, a formalism has been developed which reduces the numerical effort to the absolute minimum. The central idea of its derivation is the identification of asymptotic regions in which the recursion relations can be solved analytically. The new object in the numerical treatment is then the correction to this asymptotic regime, represented by a sequence of so-called kernel correction functions. This method increased the effciency of the numerics considerably so that up to 200 orders of RSB could be calculated at zero temperature and zero external field, and up to 60 (65) orders of RSB for finite temperature (external field). The remarkable high precision of these calculations allowed the extraction of several quantities with accuracy exceeding the literature values by several orders of magnitude. The results of the numerical calculations have been analyzed in great detail. Especially the convergence behavior of various observables and of the order function with respect to the RSB order has been investigated since the high but finite RSB regime has been addressed in the present work for the first time. Several unexpected features of finite order replica symmetry breaking have been observed.}, subject = {Spin-Spin-Wechselwirkung}, language = {en} } @phdthesis{Maier2008, author = {Maier, Andreas}, title = {Adaptively Refined Large-Eddy Simulations of Galaxy Clusters}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-32274}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2008}, abstract = {It is aim of this work to develop, implement, and apply a new numerical scheme for modeling turbulent, multiphase astrophysical flows such as galaxy cluster cores and star forming regions. The method combines the capabilities of adaptive mesh refinement (AMR) and large-eddy simulations (LES) to capture localized features and to represent unresolved turbulence, respectively; it will be referred to as Fluid mEchanics with Adaptively Refined Large-Eddy SimulationS or FEARLESS.}, subject = {Turbulenz}, language = {en} } @phdthesis{Weber2008, author = {Weber, Sebastian}, title = {Simulation of self-assembled nanopatterns in binary alloys on the fcc(111) surface}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-27914}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2008}, abstract = {In this PhD thesis, we study the heteroepitaxial crystal growth by means of Monte Carlo simulations. Of particular interest in this work is the influence of the lattice mismatch of the adsorbates relative to the substrate on surface structures. In the framework of an off-lattice model, we consider one monolayer of adsorbate and investigate the emerging nanopatterns in equilibrium and their formation during growth. In chapter 1, a brief introduction is given, which describes the role of computer simulations in the field of the physics of condensed matter. Chapter 2 is devoted to some technical basics of experimental methods of molecular beam epitaxy and the theoretical description. Before a model for the simulation can be designed, it is necessary to make some considerations of the single processes which occur during epitaxial growth. For that purpose we look at an experimental setup and extract the main microscopic processes. Afterwards a brief overview of different theoretical concepts describing that physical procedures is given. In chapter 3, the model used in the simulations is presented. The aim is to investigate the growth of an fcc crystal in the [111] direction. In order to keep the simulation times within a feasible limit a simple pair potential, the Lennard-Jones potential, with continuous particle positions is used, which are necessary to describe effects resulting from the atomic mismatch in the crystal. Furthermore the detailed algorithm is introduced which is based on the idea to calculate the barrier of each diffusion event and to use the barriers in a rejection-free method. Chapter 4 is attended to the simulation of equilibrium. The influence of different parameters on the emerging structures in the first monolayer upon the surface, which is completely covered with two adsorbate materials, is studied. Especially the competition between binding energy and strain leads to very interesting pattern formations like islands or stripes. In chapter 5 the results of growth simulations are presented. At first, we introduce a model in order to realize off-lattice Kinetic Monte Carlo simulations. Since the costs in simulation time are enormous, some simplifications in the calculation of diffusion barriers are necessary and therefore the previous model is supplemented with some elements from the so-called ball and spring model. The next point is devoted to the calculation of energy barriers followed by the presentation of the growth simulations. Binary systems with only one sort of adsorbate are investigated as well as ternary systems with two different adsorbates. Finally, a comparison to the equilibrium simulations is drawn. Chapter 6 contains some concluding remarks and gives an outlook to possible further investigations.}, subject = {Kristallwachstum}, language = {en} } @phdthesis{Walther2008, author = {Walther, Markus}, title = {Simulation of strain-induced and defect-controlled self-organization of nanostructures}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-27931}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2008}, abstract = {In this PhD thesis, the effect of strain on heteroepitaxial growth is investigated by means of Kinetic Monte Carlo simulations. In this context the lattice misfit, arising from the different lattice constants of the adsorbate and the substrate material, is of particular interest. As a consequence, this lattice misfit leads to long-range elastic strain effects having strong influence on the entire growing crystal and its resulting surface morphology. The main focus of this work is the investigation of different strain relaxation mechanisms and their controlling parameters, revealing interesting consequences on the subsequent growth. Since epitaxial growth is carried out under conditions far away from thermodynamic equilibrium, it is strongly determined by surface kinetics. At this point the relevant kinetic microscopic processes are described, followed by theoretical considerations of heteroepitaxial growth disclosing an overview over several independent methodological streams, used to model epitaxy in different time and length scales, as well as the characterization of misfit dislocations and the classification of epitaxial growth modes based on thermodynamic considerations. The epitaxial growth is performed by means of Kinetic Monte Carlo simulations which allows for the consideration of long range effects in systems with lateral extension of few hundred atoms. By using an off-lattice simulation model the particles are able to leave their predefined lattice sites, which is an indispensable condition for simulating strain relaxation mechanisms. The main idea of our used model is calculating the activation energy of all relevant thermally activated processes by using simple pair potentials and then realizing the dynamics by performing each event according to its probability by means of a rejection-free algorithm method. In addition, the crystal relaxation procedure, the grid-based particle access method, which accelerates the simulation enormously, and the efficient implementation of the algorithm are discussed. To study the influence of long range elastic strain effects, the main part of this work was realized on the two dimensional triangular lattice, which can be treated as a cross section of the real three dimensional case. Chapter 4 deals with the formation of misfit dislocations as a strain relaxation mechanism and the resulting consequences on the subsequent heteroepitaxial growth. We can distinguish between two principally different dislocation formation mechanisms, depending strongly on the sign as well as on the magnitude of the misfit, but also the surface kinetics need to be taken into account. Additionally, the dislocations affect the lattice spacings of the crystal whose observed progression is in qualitative good agreement with experimental results. Furthermore, the dislocations influence the subsequent growth of the adsorbate film, since the potential energy of an adatom is modulated by buried dislocations. A clear correlation between the lateral positions of buried dislocations and the positions of mounds grown on the surface can be observed. In chapter 5, an alternative strain relaxation mechanism is studied: the formation of three dimensional islands enables the particles to approach their preferred lattice spacing. We demonstrate that it is possible to adjust within our simulation model each of the three epitaxial growth modes: Volmer-Weber, Frank-van der Merve or layer-by-layer, and Stranski-Krastanov growth mode. Moreover, we can show that the emerging growth mode depends in principle on two parameters: on the one hand the interaction strength of adsorbate particles with each other, compared to the interaction of adsorbate with substrate particles, and on the other hand the lattice misfit between adsorbate and substrate particles. A sensible choice of these two parameters allows the realization of each growth mode within the simulations. In conclusion, the formation of nanostructures controlled by an underlying dislocation network can be applied in the concept of self-organized pattern formation as well as by the tendency to form ordered arrays of strain-induced three dimensional grown islands. In chapter 6, we extend our model to three dimensions and investigate the effect of strain on growth on bcc(100) surfaces. We introduce an anisotropic potential yielding a stable bcc lattice structure within the off-lattice representation. We can show that the strain built up in submonolayer islands is mainly released at the island edges and the lattice misfit has strong influence on the diffusion process on the plane surface as well as on the situation at island edges with eminent consequences on the appearance of submonolayer islands.}, subject = {Kristallwachstum}, language = {en} } @phdthesis{Hupp2008, author = {Hupp, Markus}, title = {Simulating Star Formation and Turbulence in Models of Isolated Disk Galaxies}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-34510}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2008}, abstract = {We model Milky Way like isolated disk galaxies in high resolution three-dimensional hydrodynamical simulations with the adaptive mesh refinement code Enzo. The model galaxies include a dark matter halo and a disk of gas and stars. We use a simple implementation of sink particles to measure and follow collapsing gas, and simulate star formation as well as stellar feedback in some cases. We investigate two largely different realizations of star formation. Firstly, we follow the classical approach to transform cold, dense gas into stars with an fixed efficiency. These kind of simulations are known to suffer from an overestimation of star formation and we observe this behavior as well. Secondly, we use our newly developed FEARLESS approach to combine hydrodynamical simulations with a semi-analytic modeling of unresolved turbulence and use this technique to dynamically determine the star formation rate. The subgrid-scale turbulence regulated star formation simulations point towards largely smaller star formation efficiencies and henceforth more realistic overall star formation rates. More work is necessary to extend this method to account for the observed highly supersonic turbulence in molecular clouds and ultimately use the turbulence regulated algorithm to simulate observed star formation relations.}, subject = {Astrophysik}, language = {en} } @phdthesis{Balzer2008, author = {Balzer, Matthias}, title = {F{\"u}llungs- und wechselwirkungsabh{\"a}ngiger Mott-{\"U}bergang: Quanten-Cluster-Rechnungen im Rahmen der Selbstenergiefunktional-Theorie}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-35266}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2008}, abstract = {Die Untersuchung stark korrelierter Elektronensysteme anhand des zweidimensionalen Hubbard-Modells bildet das zentrale Thema dieser Arbeit. Wir analysieren das Schicksal des Mott-Isolators bei Dotierung als auch bei Reduzierung der Wechselwirkungsst{\"a}rke. Die numerische Auswertung erfolgt mit Hilfe von Quanten-Cluster-Approximationen, die eine thermodynamisch konsistente Beschreibung der Grundzustandseigenschaften garantieren. Der hier verwendete Rahmen der Selbstenergiefunktional-Theorie bietet eine große Flexibilit{\"a}t bei der Konstruktion von Cluster-N{\"a}herungen. Eine detaillierte Analyse gibt Aufschluss {\"u}ber die Qualit{\"a}t und das Konvergenzverhalten unterschiedlicher Cluster-N{\"a}herungen innerhalb der Selbstenergiefunktional-Theorie. Wir verwenden f{\"u}r diese Untersuchungen das eindimensionale Hubbard-Modell und vergleichen unsere Resultate mit der exakten L{\"o}sung. In zwei Dimensionen finden wir als Grundzustand des Teilchen-Loch-symmetrischen Modells bei Halbf{\"u}llung einen antiferromagnetischen Isolator unabh{\"a}ngig von der Wechselwirkungsst{\"a}rke. Die Ber{\"u}cksichtigung kurzreichweitiger r{\"a}umlicher Korrelationen durch unsere Cluster-N{\"a}herung f{\"u}hrt, im Vergleich mit der dynamischen Mean-Field-Theorie, zu einer deutlichen Verbesserung des antiferromagnetischen Ordnungsparameters. Dar{\"u}berhinaus beobachten wir in der paramagnetischen Phase einen Metall-Isolator-{\"U}bergang als Funktion der Wechselwirkungsst{\"a}rke, der sich qualitativ vom reinen Mean-Field-Szenario unterscheidet. Ausgehend vom antiferromagnetischen Mott-Isolator zeigt sich ein f{\"u}llungsgetriebener Metall-Isolator-{\"U}bergang in eine paramagnetische metallische Phase. Abh{\"a}ngig von der verwendeten Cluster-Approximation tritt dabei zun{\"a}chst eine antiferromagnetische metallische Phase auf. Neben langreichweitiger antiferromagnetischer Ordnung haben wir in unseren Rechnungen auch Supraleitung ber{\"u}cksichtigt. Das Verhalten des supraleitenden Ordnungsparameters als Funktion der Dotierung ist dabei in guter {\"U}bereinstimmung sowohl mit anderen numerischen Verfahren als auch mit experimentellen Ergebnissen.}, subject = {Festk{\"o}rpertheorie}, language = {de} } @phdthesis{Dorner2008, author = {Dorner, Daniela}, title = {Observations of PG 1553+113 with the MAGIC telescope}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-28196}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2008}, abstract = {Blazars are among the most luminous sources in the universe. Their extreme short-time variability indicates emission processes powered by a supermassive black hole. With the current generation of Imaging Air Cherenkov Telescopes, these sources are explored at very high energies. Lowering the threshold below 100 GeV and improving the sensitivity of the telescopes, more and more blazars are discovered in this energy regime. For the MAGIC telescope, a low energy analysis has been developed allowing to reach energies of 50 GeV for the first time. The method is presented in this thesis at the example of PG 1553+113 measuring a spectrum between 50 GeV and 900 GeV. In the energy regime observed by MAGIC, strong attenuation of the gamma-rays is expected from pair production due to interactions of gamma-rays with low-energy photons from the extragalactic background light. For PG 1553+113, this provides the possibility to constrain the redshift of the source, which is still unknown. Well studied from radio to x-ray energies, PG 1553+113 was discovered in 2005 in the very high energy regime. In total, it was observed with the MAGIC telescope for 80~hours between April 2005 and April 2007. From more than three years of data taking, the MAGIC telescope provides huge amounts of data and a large number of files from various sources. To handle this data volume and to provide monitoring of the data quality, an automatic procedure is essential. Therefore, a concept for automatic data processing and management has been developed. Thanks to its flexibility, the concept is easily applicable to future projects. The implementation of an automatic analysis is running stable since three years in the data center in W{\"u}rzburg and provides consistent results of all MAGIC data, i.e. equal processing ensures comparability. In addition, this database controlled system allows for easy tests of new analysis methods and re-processing of all data with a new software version at the push of a button. At any stage, not only the availability of the data and its processing status is known, but also a large set of quality parameters and results can be queried from the database, facilitating quality checks, data selection and continuous monitoring of the telescope performance. By using the automatic analysis, the whole data sample can be analyzed in a reasonable amount of time, and the analyzers can concentrate on interpreting the results instead. For PG 1553+113, the tools and results of the automatic analysis were used. Compared to the previously published results, the software includes improvements as absolute pointing correction, absolute light calibration and improved quality and background-suppression cuts. In addition, newly developed analysis methods taking into account timing information were used. Based on the automatically produced results, the presented analysis was enhanced using a special low energy analysis. Part of the data were affected by absorption due to the Saharan Air Layer, i.e. sanddust in the atmosphere. Therefore, a new method has been developed, correcting for the effect of this meteorological phenomenon. Applying the method, the affected data could be corrected for apparent flux variations and effects of absorption on the spectrum, allowing to use the result for further studies. This is especially interesting, as these data were taken during a multi-wavelength campaign. For the whole data sample of 54 hours after quality checks, a signal from the position of PG 1553+113 was found with a significance of 15 standard deviations. Fitting a power law to the combined spectrum between 75 GeV and 900 GeV, yields a spectral slope of 4.1 +/- 0.2. Due to the low energy analysis, the spectrum could be extended to below 50 GeV. Fitting down to 48 GeV, the flux remains the same, but the slope changes to 3.7 +/- 0.1. The determined daily light curve shows that the integral flux above 150 GeV is consistent with a constant flux. Also for the spectral shape no significant variability was found in three years of observations. In July 2006, a multi-wavelength campaign was performed. Simultaneous data from the x-ray satellite Suzaku, the optical telescope KVA and the two Cherenkov experiments MAGIC and H.E.S.S. are available. Suzaku measured for the first time a spectrum up to 30 keV. The source was found to be at an intermediate flux level compared to previous x-ray measurements, and no short time variability was found in the continuous data sample of 41.1 ksec. Also in the gamma regime, no variability was found during the campaign. Assuming a maximum slope of 1.5 for the intrinsic spectrum, an upper limit of z < 0.74 was determined by deabsorbing the measured spectrum for the attenuation of photons by the extragalactic background light. For further studies, a redshift of z = 0.3 was assumed. Collecting various data from radio, infrared, optical, ultraviolet, x-ray and gama-ray energies, a spectral energy distribution was determined, including the simultaneous data of the multi-wavelength campaign. Fitting the simultaneous data with different synchrotron-self-compton models shows that the observed spectral shape can be explained with synchrotron-self-compton processes. The best result was obtained with a model assuming a log-parabolic electron distribution.}, subject = {Aktiver galaktischer Kern}, language = {en} } @phdthesis{Meyer2008, author = {Meyer, Markus}, title = {Observations of a systematically selected sample of high frequency peaked BL Lac objects with the MAGIC telescope}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-28115}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2008}, abstract = {At the beginning of regular observations with the MAGIC telescope in December 2004, all but one extragalactic sources detected at very high energy (VHE) gamma-rays belonged to the class of high frequency peaked BL Lac (HBL) objects. This motivated a systematic scan of candidate sources to increase the number of known sources and to study systematically their spectral properties. As candidate sources for VHE emission, X-ray bright HBLs were selected from a compilation of active galactic nuclei. The MAGIC observations took place from December 2004 to March 2006. The declination of the objects was restricted to values between -1.2° and +58.8° corresponding to a maximum zenith distance lower than 30° at culmination. Since gamma-rays are absorbed by photo-pair production in low energy background radiation fields, the redshift of the investigated objects was limitetd to z < 0.3. Under the assumption that HBLs generally emit the same energy flux at 1keV as at 200GeV, only the brightest X-ray sources were observed, leading to a cut in the X-ray flux of F(1keV) > 2µJy}. Of the fourteen sources observed, four have been detected: 1ES 1218+304 (for the first time at very high energies), 1ES 2344+514 (strong detection in a state of low activity), Mrk 421 and Mrk 501. A hint of a signal on a 3-sigma-level from the direction of 1ES 1011+496 has been observed. In the meantime the object has been confirmed as a source of VHE gamma-rays by a second MAGIC observation campaign triggered by an optical outburst. For ten sources, upper limits on their integral fluxes above 200GeV have been calculated on a 99\% confidence level. To cross calibrate the different data samples, collected during 14 months, bright muon ring images have been used, recorded as background events by the MAGIC telescope. Based on the development by Meyer (2003), the method has been improved and implemented into the automatic data analysis as a continuous monitor of the calibration and the point spread function of the optical system. While the ring images are generated by muons with small impact parameters, it could be shown that the image parameter distributions for muons with large impact parameters and gamma showers completely overlap, revealing these muons as the dominant background for gamma-ray observations below energies of 150GeV. The sample of HBLs (including all HBLs detected at VHE so far) has been investigated for correlations between broad-band spectral indices as determined from simultaneous optical, archival X-ray and radio luminosities, finding that the VHE emitting HBLs do not differ from the non-detected ones. In general the absorption corrected HBL gamma-ray luminosities at 200GeV are not higher than their X-ray luminosities at 1keV. Based on a complete X-ray BL Lac sample, the Hamburg/ROSAT X-ray BL Lac sample, the number of expected VHE sources has been estimated for the performed scan, finding a consistent number under the assumption of a 37\% completeness of the investigated sample and a 1keV-to-200GeV luminosity ratio of 1.4. An upper limit on the omnidirectional flux at 200GeV has been calculated by interpolating the sum over the observed fluxes and upper limits. Within the uncertainties, the result is in agreement with the expectations derived from the X-ray luminosity function of BL Lacs. For 1ES 1218+304 and 1ES 2344+514 the lightcurves have been derived, showing evidence for flux variability on a time scale of 17 days and 24h, respectively. In the case of 1ES 1218+304 variability has been reported for the first time at VHEs. For both sources the energy spectra have been reconstructed and discussed in the context of their broad band spectral energy distribution (SED), using a single zone synchrotron self Compton model. The SEDs are well fitted by the simulation even though the very high peak frequencies at gamma-rays push the model to its limits. The parameters derived from the simulation are in good agreement with the parameters found for similar HBLs.}, subject = {Aktiver galaktischer Kern}, language = {en} } @phdthesis{Koslowski2008, author = {Koslowski, Tim Andreas}, title = {Cosmological Sectors in Loop Quantum Gravity}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-28244}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2008}, abstract = {This thesis is concerned with the description of macroscopic geometries through Loop Quantum Gravity, and there particularly with the description of cosmology within full Loop Quantum Gravity. For this purpose we depart from two distinct (classically virtually equivalent) ans{\"a}tze: One is phase space reduction and the other is the restriction to particular states. It turns out that the quantum analogue of these two approaches are fundamentally different: The quantum analogue of phase space reduction needs the reformulation in terms of the observable Poisson algebra, so it can be applied to the noncommutative quantum phase space: It rests on the observation that the observable Poisson algebra of classical canonical cosmology is induced by the embedding of the reduced cosmological phase space into the phase space of full General Relativity. Using techniques related to Rieffel-induction, we develop a construction for a noncommutative embedding that has a classical limit that is described by a Poisson embedding. To be able to use this class of noncommutative embeddings for Loop Quantum Gravity, one needs a complete group of diffeomorphisms for the quantum theory, which is constructed. These two results are applied to construct a quantum embedding of a cosmological sector into full Loop Quantum Gravity. The embedded cosmological sector turns out to be discrete, like standard Loop Quantum Cosmology and can be interpreted as a super-selection sector thereof; however due to pathologies of the dynamics of full Loop Quantum Gravity, one can not induce a meaningful dynamics for this cosmological sector. The quantum analogue of restricting the space of states is achieved by explicitly constructing states for Loop Quantum Gravity with smooth geometry. These states do not exist within the Hilbert space of Loop Quantum Gravity, but as states on the observable algebra of Loop Quantum Gravity. This observable algebra is built from spin network functions, area operators and a restricted set of fluxes. For this algebra to be physically complete, we needed to construct a version of Loop Quantum Geometry based on a fundamental area operator. This version of Loop Quantum Geometry is constructed. Since the smooth geometry states are not in the Hilbert space of standard Loop Quantum Gravity, we needed to calculate the Hilbert space representation that contains them using the GNS construction. This representation of the observable algebra can be illustrated as a classical condensate of geometry with quantum fluctuations thereon. Using these representations we construct a quantum-minisuperspace, which allows for an interpretation of standard Loop Quantum Cosmology in terms of these states and led us to conjecture a new approach for the implementation of dynamics for Loop Quantum Gravity.}, subject = {Gravitation}, language = {en} } @phdthesis{Wagner2008, author = {Wagner, Alexander}, title = {Production of Sleptons in e¯e¯-Collisions}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-28307}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2008}, abstract = {Supersymmetry is currently the best motivated extension of the Standard Model and will be subject to extensive studies in the upcoming generation of colliders. The e-e- mode would be a straight forward extension to the currently planed International Linear Collider, planned to operate in e+e- mode. The low background in this mode may prove advantageous in the study of CP- and Lepton Flavour Violtation. In this work a CP sensitive observable based on transverse beam polarisation is introduced and the impact of neutralino mixing on the total cross section in cas of non-vanishing CP-violtating phases is studied in representative scenarios including non-GUT scenarios. Additionally, the mixing of sleptons is studied in the context of LFV, an analytical approximation is developed, and possible background free measurements of these effects are investigated.}, subject = {Supersymmetrie}, language = {en} } @phdthesis{Hochkeppel2008, author = {Hochkeppel, Stephan}, title = {One- and Two-Particle Correlation Functions in the Dynamical Quantum Cluster Approach}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-28705}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2008}, abstract = {This thesis is dedicated to a theoretical study of the 1-band Hubbard model in the strong coupling limit. The investigation is based on the Dynamical Cluster Approximation (DCA) which systematically restores non-local corrections to the Dynamical Mean Field approximation (DMFA). The DCA is formulated in momentum space and is characterised by a patching of the Brillouin zone where momentum conservation is only recovered between two patches. The approximation works well if k-space correlation functions show a weak momentum dependence. In order to study the temperature and doping dependence of the spin- and charge excitation spectra, we explicitly extend the Dynamical Cluster Approximation to two-particle response functions. The full irreducible two-particle vertex with three momenta and frequencies is approximated by an effective vertex dependent on the momentum and frequency of the spin and/or charge excitations. The effective vertex is calculated by using the Quantum Monte Carlo method on the finite cluster whereas the analytical continuation of dynamical quantities is performed by a stochastic version of the maximum entropy method. A comparison with high temperature auxiliary field quantum Monte Carlo data serves as a benchmark for our approach to two-particle correlation functions. Our method can reproduce basic characteristics of the spin- and charge excitation spectrum. Near and beyond optimal doping, our results provide a consistent overall picture of the interplay between charge, spin and single-particle excitations: a collective spin mode emerges at optimal doping and sufficiently low temperatures in the spin response spectrum and exhibits the energy scale of the magnetic exchange interaction J. Simultaneously, the low energy single-particle excitations are characterised by a coherent quasiparticle with bandwidth J. The origin of the quasiparticle can be quite well understood in a picture of a more or less antiferromagnetic ordered background in which holes are dressed by spin-excitations to allow for a coherent motion. By increasing doping, all features which are linked to the spin-polaron vanish in the single-particle as well as two-particle spin response spectrum. In the second part of the thesis an analysis of superconductivity in the Hubbard model is presented. The superconducting instability is implemented within the Dynamical Cluster Approximation by essentially allowing U(1) symmetry breaking baths in the QMC calculations for the cluster. The superconducting transition temperature T_c is derived from the d-wave order parameter which is directly estimated on the Monte Carlo cluster. The critical temperature T_c is in astonishing agreement with the temperature scale estimated by the divergence of the pair-field susceptibility in the paramagnetic phase. A detailed study of the pseudo and superconducting gap is continued by the investigation of the local and angle-resolved spectral function.}, subject = {Festk{\"o}rpertheorie}, language = {en} } @phdthesis{Speckner2009, author = {Speckner, Christian}, title = {LHC Phenomenology of the Three-Site Higgsless Model}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-45931}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2009}, abstract = {The Three-Site Higgsless Model is alternative implementation of electroweak symmetry breaking which in the Standard Model is mediated by the Higgs mechanism. The main features of this model is the appearance of two new heavy vector resonances W' and Z' with masses > 380 GeV as well as a set of new heavy fermions (> 1.8 TeV). In this model, unitarity of the amplitudes for the scattering of longitudinal gauge bosons is maintained by the exchange of the W' and Z' up to a scale of ~2 TeV. Consistency with the electroweak precision observables from the LEP / LEP-II experiments implies an exceedingly small coupling of the new vector bosons to the light Standard Model fermions (about 3\% of the isospin gauge coupling). In this thesis, the LHC phenomenology of this scenario is explored. To this end, we calculated the couplings and widths of all the new particles and implemented the model into the Monte-Carlo eventgenerator WHIZARD / O'Mega. With this implementation, we simulated the parton-level production of the gauge boson and fermion partners in different channels possibly suitable for their discovery at the LHC. The results are presented together with an introduction to the model and a discussion of its properties. We find that, while the fermiophobic nature of the new heavy gauge bosons does make them intrinsically difficult to observe at a collider, the LHC should be able to establish the existence of both resonances and even give some hints about the properties of their couplings which would be a vital test of the consistency of such a scenario. For the heavy fermions, we find that their large mass is accompanied by relative widths of more than \$10\\%\$, making them ill-suited for a direct discovery at the LHC. Nevertheless, our simulations reveal that there is a part of parameter space where, given enough time, patience and a good understanding of detector and backgrounds, a direct discovery might be possible.}, subject = {LHC}, language = {en} } @phdthesis{Brehm2009, author = {Brehm, Sascha}, title = {Two-Particle Excitations in the Hubbard Model for High-Temperature Superconductors: A Quantum Cluster Study}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-38719}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2009}, abstract = {Two-particle excitations, such as spin and charge excitations, play a key role in high-Tc cuprate superconductors (HTSC). Due to the antiferromagnetism of the parent compound the magnetic excitations are supposed to be directly related to the mechanism of superconductivity. In particular, the so-called resonance mode is a promising candidate for the pairing glue, a bosonic excitation mediating the electronic pairing. In addition, its interactions with itinerant electrons may be responsible for some of the observed properties of HTSC. Hence, getting to the bottom of the resonance mode is crucial for a deeper understanding of the cuprate materials . To analyze the corresponding two-particle correlation functions we develop in the present thesis a new, non-perturbative and parameter-free technique for T=0 which is based on the Variational Cluster Approach (VCA, an embedded cluster method for one-particle Green's functions). Guided by the spirit of the VCA we extract an effective electron-hole vertex from an isolated cluster and use a fully renormalized bubble susceptibility chi0 including the VCA one-particle propagators.Within our new approach, the magnetic excitations of HTSC are shown to be reproduced for the Hubbard model within the relevant strong-coupling regime. Exceptionally, the famous resonance mode occurring in the underdoped regime within the superconductivity-induced gap of spin-flip electron-hole excitations is obtained. Its intensity and hourglass dispersion are in good overall agreement with experiments. Furthermore, characteristic features such as the position in energy of the resonance mode and the difference of the imaginary part of the susceptibility in the superconducting and the normal states are in accord with Inelastic Neutron Scattering (INS) experiments. For the first time, a strongly-correlated parameter-free calculation revealed these salient magnetic properties supporting the S=1 magnetic exciton scenario for the resonance mode. Besides the INS data on magnetic properties further important new insights were gained recently via ARPES (Angle-Resolved Photoemission-Spectroscopy) and Raman experiments which disclosed a quite different doping dependence of the antinodal compared to the near-nodal gap. This thesis provides an approach to the Raman response similar to the magnetic case for inspecting this gap dichotomy. In agreement with experiments and one-particle data obtained in the VCA, we recover the antinodal gap decreasing and the near-nodal gap increasing as a function of doping. Hence, our results prove the Hubbard model to account for these salient gap features. In summary, we develop a two-particle cluster approach which is appropriate for the strongly-correlated regime and contains no free parameter. Our results obtained with this new approach combined with the phase diagram and the one-particle excitations obtained in the VCA strongly constitute a Hubbard model description of HTSC cuprate materials.}, subject = {Hochtemperatursupraleiter}, language = {en} } @phdthesis{Plentinger2009, author = {Plentinger, Florian}, title = {Systematic Model Building with Flavor Symmetries}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-38077}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2009}, abstract = {Die Beobachtung von Neutrinomassen und Leptonenmischungen haben gezeigt, dass das Standard-Modell unvollst{\"a}ndig ist. Im Zuge dieser Entdeckung tauchen neue Fragestellungen auf: warum sind die Neutrinomassen so klein, wie sieht ihre Massenhierarchie aus, warum sind die Mischungen im Quark- und Leptonen-Sektor so unterschiedlich oder welche Form hat der Higgs-Sektor. Um diese Fragen zu beantworten und um zuk{\"u}nftige experimentelle Daten vorherzusagen, werden verschiedene Ans{\"a}tze betrachtet. Besonders interessant sind Grand Unified Theories, wie SU(5) oder SO(10). GUTs sind vertikale Symmetrien, da sie die SM-Teilchen in Multipletts vereinheitlichen und {\"u}blicherweise neue Elementarteilchen vorhersagen, die durch den Seesaw-Mechanismus, auf nat{\"u}rliche Weise die Kleinheit der Neutrinomassen erkl{\"a}ren. Dar{\"u}berhinaus sind auch horizontale Symmetrien, d.h. Flavor-Symmetrien, welche auf den Generationen-Raum der SM-Teilchen wirken, interessant. Sie k{\"o}nnen die Quark- und Leptonen-Massenhierarchien, sowie die unterschiedlichen Quark- und Leptonenmischungen, erkl{\"a}ren. Ausserdem beeinflussen Flavor-Symmetrien massgeblich den Higgs-Sektor und sagen bestimmte Formen von Massenmatrizen vorher. Diese hohe Vorhersagekraft machen GUTs und Flavor-Symmetrien sowohl f{\"u}r Theoretiker, als auch f{\"u}r Experimentalphysiker interessant. Solche Erweiterungen des SM k{\"o}nnen mit weiteren Konzepten wie Supersymmetrie oder extra Dimensionen kombiniert werden. Hinzu kommt, dass sie f{\"u}r gew{\"o}hnlich Auswirkungen auf die beobachtete Materie-Antimaterie Asymmetrie des Universums haben und einen dunkle Materie Kandidaten beinhalten k{\"o}nnen. Im Allgemeinen sagen sie auch die seltene Leptonenzahl verletzenden Zerf{\"a}lle mu -> e gamma, tau -> mu gamma und tau -> e gamma vorher, die stark von Experimenten eingeschr{\"a}nkt sind, aber m{\"o}glicherweise in der Zukunft beobachtet werden. In dieser Arbeit kombinieren wir all diese Zug{\"a}nge, d.h. GUTs, den Seesaw-Mechanismus und Flavor-Symmetrien. Dr{\"u}ber hinaus ist unser Anliegen einen systematischen Zugang zum Modellbau zu entwickeln und durchzuf{\"u}hren, sowie die Suche nach ph{\"a}nomenologischen Implikationen. Dies stellt eine neue Sichtweise im Modellbau dar, da es uns erlaubt bestimmte Modelle durch ihre theoretischen und ph{\"a}nomenologischen Vorhersagen zu filtern. D.h. wir k{\"o}nnen weitere Einschr{\"a}nkungen an Modelle fordern, um ein bestimmtes auszuw{\"a}hlen. Die Ergebnisse unserer Herangehensweise sind zum Beispiel mannigfaltige Leptonen-Flavor- und GUT-Modelle, ein systematischer Scan von Leptonenzahl verletzenden Prozessen, neue Massenmatrizen, eine neues Vest{\"a}ndnis der Leptonenmischungswinkel, eine Verallgemeinerung der Idee der Quark-Leptonen-Komplementarit{\"a}t theta_12=pi/4-epsilon/sqrt{2} und zum ersten Mal die QLC-Relation in einer SU(5) GUT.}, subject = {Symmetrie}, language = {en} } @phdthesis{Elbracht2009, author = {Elbracht, Oliver}, title = {Wave Extraction in Numerical Relativity}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-40672}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2009}, abstract = {Diese Arbeit konzentriert sich auf eine fundamentale Problematik der numerischen Relativit{\"a}tstheorie: Die Extraktion von Gravitationswellen in einer eich- und koordinateninvarianten Formulierung, um ein physikalisch interpretierbares Objekt zu erhalten. Es wird eine neue Methodik entwickelt, um die physikalisch relevanten Gr{\"o}ßen aus einer numerisch erzeugten Raumzeit zu extrahieren. Wir pr{\"a}sentieren eine allgemeing{\"u}ltige kanonische Formulierung der Weyl Skalare im Newman-Penrose Formalismus als eine Funktion von fundamentalen Raumzeit-Invarianten. Dadurch zeigt sich, dass mit Hilfe dieser Methodik die explizite Konstruktion eines Vierbeins vollst{\"a}ndig redundant ist. Als weiteren Schwerpunkt charakterisieren wir innerhalb des Newman-Penrose Formalismus eine spezielle Untergruppe von Tetraden, die transversen Frames. Es wird eine bisher unbekannte Verbindung zwischen den prim{\"a}r genutzen Vierbeinen f{\"u}r die Extraktion der Wellenform abgeleitet, dem Gram-Schmidt Vierbein und dem quasi-Kinnersley Vierbein. Abschliessend studieren wir die Abh{\"a}ngigkeit der Gravitationswellen eines gest{\"o}rten Schwarzen Loches vom verwendeten Vierbein. Wir berechnen die Form der Gravitationswellen in dieser Raumzeit und demonstrieren inwieweit unsere neue Methodik robustere und exaktere Ergebnisse liefert, als die gew{\"o}hnlich verwendeten Ans{\"a}tze zur Extraktion des Signals.}, subject = {Allgemeine Relativit{\"a}tstheorie}, language = {en} } @phdthesis{Berger2009, author = {Berger, Karsten}, title = {Discovery and Characterization of the first Low-Peaked and Intermediate-Peaked BL Lacertae Objects in the Very High Energy Gamma-Ray Regime}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-37431}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2009}, abstract = {20 years after the discovery of the Crab Nebula as a source of very high energy gamma-rays, the number of sources newly discovered above 100 GeV using ground-based Cherenkov telescopes has considerably grown, at the time of writing of this thesis to a total of 81. The sources are of different types, including galactic sources such as supernova remnants, pulsars, binary systems, or so-far unidentified accelerators and extragalactic sources such as blazars and radio galaxies. The goal of this thesis work was to search for gamma-ray emission from a particular type of blazars previously undetected at very high gamma-ray energies, by using the MAGIC telescope. Those blazars previously detected were all of the same type, the so-called high-peaked BL Lacertae objects. The sources emit purely non-thermal emission, and exhibit a peak in their radio-to-X-ray spectral energy distribution at X-ray energies. The entire blazar population extends from these rare, low-luminosity BL Lacertae objects with peaks at X-ray energies to the much more numerous, high-luminosity infrared-peaked radio quasars. Indeed, the low-peaked sources dominate the source counts obtained from space-borne observations at gamma-ray energies up to 10 GeV. Their spectra observed at lower gamma-ray energies show power-law extensions to higher energies, although theoretical models suggest them to turn over at energies below 100 GeV. This opened the quest for MAGIC as the Cherenkov telescope with the currently lowest energy threshold. In the framework of this thesis, the search was focused on the prominent sources BL Lac, W Comae and S5 0716+714, respectively. Two of the sources were unambiguously discovered at very high energy gamma-rays with the MAGIC telescope, based on the analysis of a total of about 150 hours worth of data collected between 2005 and 2008. The analysis of this very large data set required novel techniques for treating the effects of twilight conditions on the data quality. This was successfully achieved and resulted in a vastly improved performance of the MAGIC telescope in monitoring campaigns. The detections of low-peaked and intermediate-peaked BL Lac objects are in line with theoretical expectations, but push the models based on electron shock acceleration and inverse-Compton cooling to their limits. The short variability time scales of the order of one day observed at very high energies show that the gamma-rays originate rather close to the putative supermassive black holes in the centers of blazars, corresponding to less than 1000 Schwarzschild radii when taking into account relativistic bulk motion.}, subject = {Aktiver galaktischer Kern}, language = {en} } @phdthesis{Schutzmeier2009, author = {Schutzmeier, Thomas}, title = {Matrix elements for the B -> Xsγ decay at NNLO}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-50026}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2009}, abstract = {Einer der interssantesten Prozesse im Flavour Sektor des Standard Modells (SM) im Kontext der indirekten Suche nach neuer Physik ist der seltene inklusive Zerfall B -> Xs gamma. Dieser Zerfallskanal entspricht einem neutralen Strom mit Wechsel des Flavours zwischen Anfangs- und Endzustand. Im SM ist ein solcher Uebergang unterdrueckt, da er nur ueber Schleifenbeitraege erfolgen kann, und ist somit sensitiv auf Beitraege neuer Physik. Darueber hinaus sind nichtperturbative Beitraege moderat, was praezise theoretische Vorhersagen im Rahmen einer effektiven Niederenergie Theorie ermoeglicht. Sowohl praezise Messungen als auch genaue theoretische Vorhersagen mit einer guten Kontrolle ueber perturbative und nichtperturbative Effekte sind notwendig, um den Parameterraum von Modellen jenseits des SM einzuschraenken. Experimentell wurde die Zerfallsrate B -> Xs gamma vor Allem mit Hilfe der spezialisierten Experimente BaBar und Belle an den sogenannten B Fabriken mit einer hervorragenden Genauigkeit gemessen. Um diese Praezision auch in der theoretische Vorhersage zu erhalten, sind hoehere Ordnungen in der effektiven Stoerungstheorie essentiell. Tatsaechlich fuehrt erst die Beruecksichtigung von QCD Korrekturen auf der naechst-zu-naechst-zu hoeheren Ordnung (NNLO) in Stoerungstheorie zu einer mit dem Experiment vergleichbaren theoretischen Unsicherheit. Die Bestimmung des Verzweigungsverhaeltnisses von B -> Xs gamma auf NNLO wurde innerhalb der letzten zehn Jahre von mehreren Arbeitsgruppen angegangen. Ein Gro"steil dieses Projekts wurde abgeschlossen und eine erste Abschaetzung auf diesem Niveau der Stoerungstheorie 2006 publiziert. Allerdings standen fuer diese Vorhersage nicht alle Beitraege von nach wie vor unbekannten Matrixelementen zur Verfuegung, die nur aus partiell bekannten Resultaten abgeschaetzt werden mussten. In dieser Arbeit bereiten wir einen Rahmen fuer die systematische Bestimmung der noch nicht verfuegbaren Matrixelemente auf NNLO. Ein Hauptergebnis dieser Dissertation ist die Bestimmung von fermionischen Korrekturen zu Matrixelementen von Vier-Quark Operatoren in der effektiven Theorie. Erstmalig wird hierbei die volle Massenabhaengigkeit beruecksichtigt. Ein weiterer Schwerpunkt liegt auf der Berechnung von fermionischen als auch bosonischen Korrekturen im Grenzwert einer verschwindenden Masse des Charm Quarks. Zusammen mit noch unbekannten reellen Korrekturen werden diese Ergebnisse dazu beitragen, die Unsicherheit der NNLO Vorhersage signifikant zu reduzieren. Ein wesentlicher Bestandteil dieser Arbeit, der die hier durchgefuehrten Berechnungen erst ermoeglichte, ist die Entwicklung einer automatisierten Methode zur hochpreazisen Bestimmung von Vielschleifenintegralen die zwei Massenskalen enthalten.}, subject = {Flavour }, language = {en} } @phdthesis{Knochel2009, author = {Knochel, Alexander Karl}, title = {Supersymmetry in a Sector of Higgsless Electroweak Symmetry Breaking}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-47899}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2009}, abstract = {Seit der Popularisierung durch Randall/Sundrum (RS) vor etwa 10 Jahren, und insbesondere in Verbindung mit der \$AdS/CFT\$-Korrespondenz, ist der Einsatz von gekr{\"u}mmten Raumzeithintergr{\"u}nden mit Extradimensionen (insb. des \$AdS_5\$) eine der fruchtbarsten neuen Ideen bei der Suche nach Modellen jenseits des Standardmodells (SM). Dieser Ansatz brachte nicht nur frische Einsichten in die Physik stark wechselwirkender Feldtheorien, die zuvor st{\"o}rungstheoretischen Methoden verschlossen waren, sondern schaffte auch einen faszinierenden neuen Zusammenhang zwischen ph{\"a}nomenologi-schen Modellen an der TeV-Skala und der Gravitation. Dies hat unter anderem auch das Interesse an Modellen der elektroschwachen Symmetriebrechung ohne physikalische Skalarfelder (Higgslose Modelle'') in diesem Kontext mit dem Ziel neu aufleben lassen, Alternativen zu dem im Standardmodell der Teilchenphysik enthaltenen Higgs-Mechanismus zu finden. Bei der Umsetzung dieser Ideen lag das Hauptaugenmerk meisst auf potentiellen neuen Betr{\"a}gen zu elektroschwachen Pr{\"a}zisionsobservablen. Gleichzeitig gibt es jedoch sehr starke astrophysikalische Indizien daf{\"u}r dass die Antwort auf die Frage nach dem Ursprung der beobachteten Dunkelmaterie in Teilchenmodellen jenseits des Standardmodells zu finden ist. Die Natur der elektroschwachen Symmetriebrechung und der Dunkelmaterie geh{\"o}ren zu den zentralen Fragen deren Beantwortung dank aktueller und anstehender Experimente z.B. an Beschleunigern wie dem Tevatron wie auch in naher Zukunft am LHC in greifbare N{\"a}he r{\"u}ckt. Diese Situation legt nahe dass neue Szenarien jenseits des Standardmodells beide Fragestellungen gleicherma\ss en thematisieren sollten. In der vorliegenden Arbeit untersuchen wir die ph{\"a}nomenologischen Implikationen einer Erweiterung Higgsloser Modelle in 5D um Supersymmetrie mit erhaltener R-Parit{\"a}t im elektroschwachen Symmetriebrechungssektor. Das Ziel war, eine m{\"o}glichst einfache Erweiterung zu finden, die ein realistisches leichtes Spektrum aufweist und gleichzeitig einen guten Kandidaten f{\"u}r kalte Dunkelmaterie enth{\"a}lt, ohne zu viele freie Parameter einzuf{\"u}hren. Um dies zu bewerkstelligen, bot sich der gleiche Mechanismus an, der bereits f{\"u}r die Brechung der Eichsymmetrien zum Einsatz kommt, n{\"a}mlich die Brechung durch Randbedingungen. W{\"a}hrend Supersymmetrie in 5D vier Superladungen beinhaltet und somit eng mit \$\mathcal{N}=2\$ Supersymmetrie in 4D verwandt ist, wird allein durch den RS-Hintergrund die H{\"a}lfte der Symmetrien gebrochen, so dass nach der Kaluza-Klein-Reduktion lediglich eine erhaltene Supersymmetrie verbleibt. Davon ausgehend war das einfachste gangbare Szenario, die Brechung der verbleibenden Generatoren effektiv durch Randbedingungen auf der UV-Brane der RS-Raumzeit zu beschreiben. Obwohl hierdurch Teile des leichten SUSY-Spektrums, insb. die Superpartner der Fermionen, ausprojeziert werden, verbleibt die reichhaltige Ph{\"a}nomenologie von vollst{\"a}ndigen \$\mathcal{N}=2\$-Multiplets im Kaluza-Klein-Sektor. Das leichte erweiterte Spektrum besteht aus den Superpartnern der elektroschwachen Eichbosonen, die Massen um \$\mathcal{O}(100\mbox{ GeV})\$ erhalten. Die Neutralinos als Masseneigenzust{\"a}nde des neutralen Bino-Wino-Sektors sind automatisch die leichtesten Supersymmetrischen Teilchen (LSP) und damit nat{\"u}rliche Kandidaten f{\"u}r kalte Dunkelmaterie. Ihre Reliktdichte kann ohne exzessive Feineinstellung von Parametern in Einklang mit Beobachtungen gebracht werden. Das Modell sagt somit eine leichte NLSP-Masse im Bereich \$m_{\chi^+}\approx 100\dots 110\$ GeV und einen LSP bei etwa \$m_\chi\approx 90\$ GeV voraus. Am LHC hat der nicht-supersymmetrische Teilcheninhalt des Modells weitestgehend die gleichen ph{\"a}nomenologischen Konsequenzen wie sie bereits von Studien Higgsloser Modelle bekannt sind. Wir haben uns daher auf die Produktion des LSP und NLSP am LHC als typische Signatur des erweiterten Modells konzentriert, und insbesondere Monte-Carlo-Simulationen mit \nameomega/\namewhizard~zur Beobachtung von fehlender transversaler Energie (\$\ptmiss\$) in Assoziation mit schweren Quarks durchgef{\"u}hrt. Wir diskutieren geeignete Schnitte auf Winkel, invariante Massen und Impulse, und erhalten Hadronische Produktionsquerschnitte von \$\sigma>100\mbox{ fb}\$ bei \$14\mbox{ TeV}\$, die charakteristische \$\ptmiss\$-Verteilungen im \$\chi\chi t\overline{t}\$ Endzustand aufweisen. Der Nachweis {\"u}ber die Produktion von \$b\$-Paaren erweist sich als schwieriger. Unsere Ergebnisse legen nahe dass die Entdeckung dieses Typs von Dunkelmaterie in Higgslosen Modellen am LHC {\"u}ber fehlende transversale Energie mit wenigen fb\$^{-1}\$ bei 14 TeV m{\"o}glich ist, insofern eine zuverl{\"a}ssige Identifikation schwerer Quarks gegeben ist.}, subject = {Supersymmetrie}, language = {en} } @article{SchenkelUhlemann2010, author = {Schenkel, Alexander and Uhlemann, Christoph F.}, title = {Field Theory on Curved Noncommutative Spacetimes}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-68648}, year = {2010}, abstract = {We study classical scalar field theories on noncommutative curved spacetimes. Following the approach of Wess et al. [Classical Quantum Gravity 22 (2005), 3511 and Classical Quantum Gravity 23 (2006), 1883], we describe noncommutative spacetimes by using (Abelian) Drinfel'd twists and the associated ?-products and ?-differential geometry. In particular, we allow for position dependent noncommutativity and do not restrict ourselves to the Moyal-Weyl deformation. We construct action functionals for real scalar fields on noncommutative curved spacetimes, and derive the corresponding deformed wave equations. We provide explicit examples of deformed Klein-Gordon operators for noncommutative Minkowski, de Sitter, Schwarzschild and Randall-Sundrum spacetimes, which solve the noncommutative Einstein equations. We study the construction of deformed Green's functions and provide a diagrammatic approach for their perturbative calculation. The leading noncommutative corrections to the Green's functions for our examples are derived.}, subject = {Physik}, language = {en} } @phdthesis{CardosoBarato2010, author = {Cardoso Barato, Andre}, title = {Nonequilibrium phase transitions and surface growth}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-50122}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2010}, abstract = {This thesis is concerned with the statistical physics of various systems far from thermal equilibrium, focusing on universal critical properties, scaling laws and the role of fluctuations. To this end we study several models which serve as paradigmatic examples, such as surface growth and non-equilibrium wetting as well as phase transitions into absorbing states. As a particular interesting example of a model with a non-conventional scaling behavior, we study a simplified model for pulsed laser deposition by rate equations and Monte Carlo simulations. We consider a set of equations, where islands are assumed to be point-like, as well as an improved one that takes the size of the islands into account. The first set of equations is solved exactly but its predictive power is restricted to the first few pulses. The improved set of equations is integrated numerically, is in excellent agreement with simulations, and fully accounts for the crossover from continuous to pulsed deposition. Moreover, we analyze the scaling of the nucleation density and show numerical results indicating that a previously observed logarithmic scaling does not apply. In order to understand the impact of boundaries on critical phenomena, we introduce particle models displaying a boundary-induced absorbing state phase transition. These are one-dimensional systems consisting of a single site (the boundary) where creation and annihilation of particles occur, while particles move diffusively in the bulk. We study different versions of these models and confirm that, except for one exactly solvable bosonic variant exhibiting a discontinuous transition with trivial exponents, all the others display a non-trivial behavior, with critical exponents differing from their mean-field values, representing a universality class. We show that these systems are related to a \$(0+1)\$-dimensional non-Markovian model, meaning that in nonequilibrium a phase transition can take place even in zero dimensions, if time long-range interactions are considered. We argue that these models constitute the simplest universality class of phase transition into an absorbing state, because the transition is induced by the dynamics of a single site. Moreover, this universality class has a simple field theory, corresponding to a zero dimensional limit of direct percolation with L{\'e}vy flights in time. Another boundary phenomena occurs if a nonequilibrium growing interface is exposed to a substrate, in this case a nonequilibrium wetting transition may take place. This transition can be studied through Langevin equations or discrete growth models. In the first case, the Kardar-Parisi-Zhang equation, which defines a very robust universality class for nonequilibrium moving interfaces, is combined with a soft-wall potential. While in the second, microscopic models, in the corresponding universality class, with evaporation and deposition of particles in the presence of hard-wall are studied. Equilibrium wetting is related to a particular case of the problem, corresponding to the Edwards-Wilkinson equation with a potential in the continuum approach or to the fulfillment of detailed balance in the microscopic models. In this thesis we present the analytical and numerical methods used to investigate the problem and the very rich behavior that is observed with them. The entropy production for a Markov process with a nonequilibrium stationary state is expected to give a quantitative measure of the distance form equilibrium. In the final chapter of this thesis, we consider a Kardar-Parisi-Zhang interface and investigate how entropy production varies with the interface velocity and its dependence on the interface slope, which are quantities that characterize how far the stationary state of the interface is away from equilibrium. We obtain results in agreement with the idea that the entropy production gives a measure of the distance from equilibrium. Moreover we use the same model to study fluctuation relations. The fluctuation relation is a symmetry in the large deviation function associated to the probability of the variation of entropy during a fixed time interval. We argue that the entropy and height are similar quantities within the model we consider and we calculate the Legendre transform of the large deviation function associated to the height for small systems. We observe that there is no fluctuation relation for the height, nevertheless its large deviation function is still symmetric.}, subject = {Nichtgleichgewichtsstatistik}, language = {en} } @phdthesis{Burkart2010, author = {Burkart, Thomas}, title = {Der Einfluss des fundamentalen Massenverh{\"a}ltnisses auf die Teilchenbeschleunigung durch Plasmainstabilit{\"a}ten}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-56636}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2010}, abstract = {Im Rahmen dieser Arbeit wurde ein dreidimensionaler vollrelativistischer und parallelisierter Particle-in-Cell Code geschrieben, ausf{\"u}hrlich getestet und angewandt. Der Code ACRONYM ist variabel einsetzbar und von der Genauigkeit und Stabilit{\"a}t her State-of-the-Art und somit konkurrenzf{\"a}hig zu den sonstigen in der Astrophysik eingesetzten Codes anderer Gruppen. Die Energie bleibt bis auf einen Fehler von < 0.03\% erhalten, die Divergenz des Magnetfeldes bleibt immer unter einem Wert von 10^{-12} und die Skalierung wurde mittlerweile bis zu einem Clustergr{\"o}ße von einigen 10000 CPUs getestet. In dieser Arbeit wurde dann, nach der Entwicklung des Codes, der Einfluss des fundamentalen Massenverh{\"a}ltnisses m_p/m_e auf die Teilchenbeschleunigung durch Plasmainstabilit{\"a}ten untersucht. Dies ist relevant und wichtig, da in PiC-Simulationen in den allermeisten F{\"a}llen nicht mit dem realen Massenverh{\"a}ltnis gerechnet wird, da sonst viel zu viel Rechenleistung ben{\"o}tigt w{\"u}rde, um zu sehen, was mit den Protonen geschieht und was ihr Einfluss auf die leichten Teilchen wie Elektronen und Positronen ist. Zu diesem Zweck wurden Simulationen mit Massenverh{\"a}ltnissen zwischen m_p/m_e = 1.0 und 200.0 durchgef{\"u}hrt. Diese haben alle gemeinsam, dass periodische Randbedingungen verwendet wurden und das zur Verf{\"u}gung stehende Simulationsgebiet mit jeweils zwei gegeneinander str{\"o}menden Plasmapopulationen vollst{\"a}ndig gef{\"u}llt wurde, um jegliche Art von auftretenden Schocks auszuschließen. Die Rohdaten der einzelnen Simulationen wurden auf vielf{\"a}ltige Art und Weise analysiert, es wurden z.B. Schnitte durch die Teilchenverteilung erstellt, sowie ein- oder zweidimensionale Histogramme und Energieverl{\"a}ufe betrachtet. Dabei haben sich folgende Kernpunkte ergeben: F{\"u}r Massenverh{\"a}ltnisse bis etwa m_p/m_e = 20 bildet sich die gesamte Zweistrom-Instabilit{\"a}t in nur einer Phase aus, das heißt, es bilden sich von ringf{\"o}rmigen Magnetfeldern umgebene Flussschl{\"a}uche aus, die dann verschmelzen, bis nur noch zwei {\"u}brig sind und alle Teilchen werden {\"u}ber den gesamten Verlauf der Instabilit{\"a}t beschleunigt. Es ist damit zu folgern, dass die unterschiedlich schweren Teilchenspezies Protonen und Elektronen/Positronen durch die relativ nahe beieinander liegenden Massen noch so stark gekoppelt sind, dass sich nur eine Instabilit{\"a}t entwickeln kann. Bei großen Massenverh{\"a}ltnissen (m_p/m_e > 20) ist eine deutliche Trennung in zwei Phasen der Instabilit{\"a}t zu erkennen. Zuerst bilden sich wiederum Flussschl{\"a}uche aus, diese verschmelzen miteinander (zu zweien oder mehr), bevor der erste Teil der Instabilit{\"a}t abflaut. Anschließend entstehen wieder ringf{\"o}rmige Magnetfelder und Flussschl{\"a}uche, von denen einer meist deutlich st{\"a}rker ist als all die anderen, das bedeutet, dass dieser von st{\"a}rkeren Magnetfeldern umgeben ist und eine h{\"o}here Teilchendichte aufweist. Im Rahmen dieser zweigeteilten Instabilit{\"a}t werden die Elektronen und Positronen nur in der ersten Phase signifikant beschleunigt, die deutlich schwereren Protonen gewinnen {\"u}ber den gesamten Zeitraum Energie. Die h{\"o}chstenergetischen Teilchen erreichen im Ruhesystem der jeweiligen Plasmapopulation Werte um gamma = 250. Man kann daraus f{\"u}r zuk{\"u}nftige Untersuchungen mit Hilfe von Particle-in-Cell Codes den Schluss ziehen, dass R{\"u}ckschl{\"u}sse auf das tats{\"a}chliche Verhalten beim realen Massenverh{\"a}ltnis von m_p/m_e = 1836.2 nur aus den Simulationen mit m_p/m_e >> 20 gezogen werden k{\"o}nnen, da die starke Kopplung der leichten und schweren Teilchen bei kleineren Massenverh{\"a}ltnissen die Ergebnisse sehr stark beeinflusst. Es wurde anhand der gemessenen Zeitpunkte der Instabilit{\"a}tsmaxima eine Extrapolation durchgef{\"u}hrt, die zeigt, dass die Instabilit{\"a}t beim realen Massenverh{\"a}ltnis etwa bei t = 1400 omega_{pe}^{-1} auftreten w{\"u}rde. Um dies wirklich zu simulieren m{\"u}sste allerdings mehr als die 1000-fache Anzahl an CPU-Stunden aufgewandt werden. Des weiteren wurde eine Maxwell-J{\"u}ttner-Verteilung an die Teilchenverteilungen der einzelnen Simulationen auf dem H{\"o}hepunkt der Instabilit{\"a}t gefittet, um sowohl die neue Temperatur des Plasmas als auch die Beschleunigungseffizienz des Prozesses zu berechnen. Die Temperatur erh{\"o}ht sich demnach durch die Instabilit{\"a}t von etwa 10^8K auf 10^{10} bis 10^{11}K, der Anteil suprathermischer Teilchen betr{\"a}gt 2 bis 4\%.}, subject = {Astrophysik}, language = {de} } @phdthesis{Lang2010, author = {Lang, Thomas C.}, title = {Quantum Monte Carlo methods and strongly correlated electrons on honeycomb structures}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-53506}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2010}, abstract = {In this thesis we apply recently developed, as well as sophisticated quantum Monte Carlo methods to numerically investigate models of strongly correlated electron systems on honeycomb structures. The latter are of particular interest owing to their unique properties when simulating electrons on them, like the relativistic dispersion, strong quantum fluctuations and their resistance against instabilities. This work covers several projects including the advancement of the weak-coupling continuous time quantum Monte Carlo and its application to zero temperature and phonons, quantum phase transitions of valence bond solids in spin-1/2 Heisenberg systems using projector quantum Monte Carlo in the valence bond basis, and the magnetic field induced transition to a canted antiferromagnet of the Hubbard model on the honeycomb lattice. The emphasis lies on two projects investigating the phase diagram of the SU(2) and the SU(N)-symmetric Hubbard model on the hexagonal lattice. At sufficiently low temperatures, condensed-matter systems tend to develop order. An exception are quantum spin-liquids, where fluctuations prevent a transition to an ordered state down to the lowest temperatures. Previously elusive in experimentally relevant microscopic two-dimensional models, we show by means of large-scale quantum Monte Carlo simulations of the SU(2) Hubbard model on the honeycomb lattice, that a quantum spin-liquid emerges between the state described by massless Dirac fermions and an antiferromagnetically ordered Mott insulator. This unexpected quantum-disordered state is found to be a short-range resonating valence bond liquid, akin to the one proposed for high temperature superconductors. Inspired by the rich phase diagrams of SU(N) models we study the SU(N)-symmetric Hubbard Heisenberg quantum antiferromagnet on the honeycomb lattice to investigate the reliability of 1/N corrections to large-N results by means of numerically exact QMC simulations. We study the melting of phases as correlations increase with decreasing N and determine whether the quantum spin liquid found in the SU(2) Hubbard model at intermediate coupling is a specific feature, or also exists in the unconstrained t-J model and higher symmetries.}, subject = {Monte-Carlo-Simulation}, language = {en} } @phdthesis{HoehneMoench2010, author = {H{\"o}hne-M{\"o}nch, Daniel}, title = {Steady-state emission of blazars at very high energies}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-53700}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2010}, abstract = {One key scientific program of the MAGIC telescope project is the discovery and detection of blazars. They constitute the most prominent extragalactic source class in the very high energy (VHE) Gamma-ray regime with 29 out of 34 known objects (as of April 2010). Therefore a major part of the available observation time was spent in the last years on high-frequency peaked blazars. The selection criteria were chosen to increase the detection probability. As the X-ray flux is believed to be correlated to the VHE Gamma-ray flux, only X-ray selected sources with a flux F(X) > 2 μJy at 1 keV were considered. To avoid strong attenuation of the Gamma-rays in the extragalactic infrared background, the redshift was restricted to values between z < 0.15 and z < 0.4, depending on the declination of the objects. The latter determines the zenith distance during culmination which should not exceed 30° (for z < 0.4) and 45° (for z < 0.15), respectively. Between August 2005 and April 2009, a sample of 24 X-ray selected high-frequency peaked blazars has been observed with the MAGIC telescope. Three of them were detected including 1ES 1218+304 being the first high-frequency peaked BL Lacertae object (HBL) to be discovered with MAGIC in VHE Gamma-rays. One previously detected object was not confirmed as VHE emitter in this campaign by MAGIC. A set of 20 blazars previously not detected will be treated more closely in this work. In this campaign, during almost four years ~ 450 hrs or ~ 22\% of the available observation time for extragalactic objects were dedicated to investigate the baseline emission of blazars and their broadband spectral properties in this emission state. For the sample of 20 objects in a redshift range of 0.018 < z < 0.361 integral flux upper limits in the VHE range on the 99.7\% confidence level (corresponding to 3 standard deviations) were calculated resulting in values between 2.9\% and 14.7\% of the integral flux of the Crab Nebula. As the distribution of significances of the individual objects shows a clear shift to positive values, a stacking method was applied to the sample. For the whole set of 20 objects, an excess of Gamma-rays was found with a significance of 4.5 standard deviations in 349.5 hours of effective exposure time. For the first time a signal stacking in the VHE regime turned out to be successful. The measured integral flux from the cumulative signal corresponds to 1.4\% of the Crab Nebula flux above 150 GeV with a spectral index α = -3.15±0.57. None of the objects showed any significant variability during the observation time and therefore the detected signal can be interpreted as the baseline emission of these objects. For the individual objects lower limits on the broad-band spectral indices αX-Gamma between the X-ray range at 1 keV and the VHE Gamma-ray regime at 200 GeV were calculated. The majority of objects show a spectral behaviour as expected from the source class of HBLs: The energy output in the VHE regime is in general lower than in X-rays. For the stacked blazar sample the broad-band spectral index was calculated to αX-Gamma = 1.09, confirming the result found for the individual objects. Another evidence for the revelation of the baseline emission is the broad-band spectral energy distribution (SED) comprising archival as well as contemporaneous multi-wavelength data from the radio to the VHE band. The SEDs of known VHE Gamma-ray sources in low flux states matches well the SED of the stacked blazar sample.}, subject = {MAGIC-Teleskop}, language = {en} } @phdthesis{Staub2010, author = {Staub, Florian}, title = {Considerations on supersymmetric Dark Matter beyond the MSSM}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-55343}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2010}, abstract = {The standard model (SM) of particle physics is for the last three decades a very successful description of the properties and interactions of all known elementary particles. Currently, it is again probed with the first collisions at the Large Hadron Collider (LHC). It is widely expected that new physics will be detected at the LHC and the SM has to be extended. The most exhaustive analyzed extension of the SM is supersymmetry (SUSY). SUSY can not only solve intrinsic problems of the SM like the hierarchy problem, but it also postulates new particles which might explain the nature of dark matter in the universe. The majority of all studies about dark matter in the framework of SUSY has focused on the minimal supersymmetric standard model (MSSM). The aim of this work is to consider scenarios beyond that scope. We consider two models which explain not only dark matter but also neutrino masses: the gravitino as dark matter in gauge mediated SUSY breaking (GMSB) with bilinear broken \$R\$-parity as well as different seesaw scenarios with the neutralino as dark matter candidate. Furthermore, we also study the next-to-minimal supersymmetric standard model (NMSSM) which solves the \(\mu\)-problem of the MSSM and discuss the properties of the neutralino as dark matter candidate. In case of \$R\$-parity violation, light gravitinos are often the only remaining candidate for dark matter in SUSY because of their very long life time. We reconsider the cosmological gravitino problem arising for this kind of models. It will be shown that the proposed solution for the overclosure of the universe by light gravitinos, namely the entropy production by decays of GMSB messenger, just works in a small subset of models and in fine-tuned regions of the parameter space. This is a consequence of two effects so far overlooked: the enhanced decay channels in massive vector bosons and the impact of charged messenger particles. Both aspects cause an interplay between different cosmological restrictions which lead to strong constraints on the parameters of GMSB models. Afterwards, a minimal supergravity (mSugra) scenario with additional chiral superfields at high energy scales is considered. These fields are arranged in complete \$SU(5)\$ multiplets in order to maintain gauge unification. The new fields generate a dimension 5 operator to explain neutrino data. Furthermore, they cause large differences in mass spectrum of MSSM fields because of the different evaluation of the renormalization group equations what changes also the properties of the lightest neutralino as dark matter candidate. We discuss the parameter space of all three possible seesaw scenarios with respect to dark matter and the impact on rare lepton flavor violating processes. As we will see, especially in seesaw type~III but also in type~II the mass spectrum and regions of parameter space consistent with dark matter differ significantly in comparison to a common mSugra scenario. Moreover, the experimental bounds, in particular of branching ratios like \(l_i \rightarrow l_j \gamma\), cause large constraints on the seesaw parameters.}, subject = {Supersymmetrie}, language = {en} } @phdthesis{Martin2010, author = {Martin, Lee C.}, title = {The Kondo Lattice Model: a Dynamical Cluster Approximation Approach}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-49446}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2010}, abstract = {We apply an antiferromagnetic symmetry breaking implementation of the dynamical cluster approximation (DCA) to investigate the two-dimensional hole-doped Kondo lattice model (KLM) with hopping \$t\$ and coupling \$J\$. The DCA is an approximation at the level of the self-energy. Short range correlations on a small cluster, which is self-consistently embedded in the remaining bath electrons of the system, are handled exactly whereas longer ranged spacial correlations are incorporated on a mean-field level. The dynamics of the system, however, are retained in full. The strong temporal nature of correlations in the KLM make the model particularly suitable to investigation with the DCA. Our precise DCA calculations of single particle spectral functions compare well with exact lattice QMC results at the particle-hole symmetric point. However, our DCA version, combined with a QMC cluster solver, also allows simulations away from particle-hole symmetry and has enabled us to map out the magnetic phase diagram of the model as a function of doping and coupling \$J/t\$. At half-filling, our results show that the linear behaviour of the quasi-particle gap at small values of \$J/t\$ is a direct consequence of particle-hole symmetry, which leads to nesting of the Fermi surface. Breaking the symmetry, by inclusion of a diagonal hopping term, results in a greatly reduced gap which appears to follow a Kondo scale. Upon doping, the magnetic phase observed at half-filling survives and ultimately gives way to a paramagnetic phase. Across this magnetic order-disorder transition, we track the topology of the Fermi surface. The phase diagram is composed of three distinct regions: Paramagnetic with {\it large} Fermi surface, in which the magnetic moments are included in the Luttinger sum rule, lightly antiferromagnetic with large Fermi surface topology, and strongly antiferromagnetic with {\it small} Fermi surface, where the magnetic moments drop out of the Luttinger volume. We draw on a mean-field Hamiltonian with order parameters for both magnetisation and Kondo screening as a tool for interpretation of our DCA results. Initial results for fixed coupling and doping but varying temperature are also presented, where the aim is look for signals of the energy scales in the system: the Kondo temperature \$T_{K}\$ for initial Kondo screening of the magnetic moments, the Neel temperature \$T_{N}\$ for antiferromagnetic ordering, a possible \$T^{*}\$ at which a reordering of the Fermi surface is observed, and finally, the formation of the coherent heavy fermion state at \$T_{coh}\$.}, subject = {Gittermodell}, language = {en} } @article{PinkertSchultzReichardt2010, author = {Pinkert, Stefan and Schultz, Joerg and Reichardt, Joerg}, title = {Protein Interaction Networks-More Than Mere Modules}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-68426}, year = {2010}, abstract = {It is widely believed that the modular organization of cellular function is reflected in a modular structure of molecular networks. A common view is that a ''module'' in a network is a cohesively linked group of nodes, densely connected internally and sparsely interacting with the rest of the network. Many algorithms try to identify functional modules in protein-interaction networks (PIN) by searching for such cohesive groups of proteins. Here, we present an alternative approach independent of any prior definition of what actually constitutes a ''module''. In a self-consistent manner, proteins are grouped into ''functional roles'' if they interact in similar ways with other proteins according to their functional roles. Such grouping may well result in cohesive modules again, but only if the network structure actually supports this. We applied our method to the PIN from the Human Protein Reference Database (HPRD) and found that a representation of the network in terms of cohesive modules, at least on a global scale, does not optimally represent the network's structure because it focuses on finding independent groups of proteins. In contrast, a decomposition into functional roles is able to depict the structure much better as it also takes into account the interdependencies between roles and even allows groupings based on the absence of interactions between proteins in the same functional role. This, for example, is the case for transmembrane proteins, which could never be recognized as a cohesive group of nodes in a PIN. When mapping experimental methods onto the groups, we identified profound differences in the coverage suggesting that our method is able to capture experimental bias in the data, too. For example yeast-two-hybrid data were highly overrepresented in one particular group. Thus, there is more structure in protein-interaction networks than cohesive modules alone and we believe this finding can significantly improve automated function prediction algorithms.}, subject = {Netzwerk}, language = {en} } @phdthesis{Paul2010, author = {Paul, Surajit}, title = {Evolution of shocks and turbulence in major galaxy-cluster mergers}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-47266}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2010}, abstract = {Mergers between rich clusters of galaxies represent the most violent events in the Universe. The merger events initiate a complex chain of processes that leads to the dissipation of the collisional energy. This phase of violent relaxation is accompanied by turbulence and shock waves as well as non-thermal particle acceleration. This thesis aims at the interpretation of multi-wavelength observations of the merging cluster of galaxies Abell 3376 in the framework of a theoretical model of the involved effects. Observations with the Very Large Array radio interferometer were carried out and analyzed to clarify the morphology of the non-thermal particle distribution in Abell 3376, in particular about the shocked regions. The dissipation in the hot intra-cluster gas was studied using archival X-ray observations with ROSAT and XMM. Results were compared with constrained numerical simulations of the evolution of the merger process in the framework of cosmological structure formation. For this purpose, the ENZO-Code was employed for the computation of the gas dynamics and self-gravity of the colliding mass distribution. The non-thermal properties of the intra-cluster gas could be indirectly inferred from the local Mach number and the strength of the turbulence.}, subject = {Galaxienhaufen}, language = {en} } @phdthesis{Edelhaeuser2011, author = {Edelh{\"a}user, Lisa}, title = {Model Independent Spin Determination at Hadron Colliders}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-71030}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2011}, abstract = {Mit dem Ende des Jahres 2011 haben die beiden LHC-Experimente ATLAS und CMS jeweils ungef\"ahr 5 inverse Femtobarn an Daten bei einer Energie von 7 TeV aufgenommen. Die bisher analysierten Daten geben nur sehr vage Hinweise auf neue Physik an der TeV-Skala. Trotzdem erwartet man, dass sich an dieser Skala neue Physik zeigt, die bekannte Probleme des Standardmodells behebt. In den letzten Jahrzehnten wurden viele Erweiterungen des Standardmodells der Teilchenphysik und ihre Ph\"anomenologie dazu ausgearbeitet. Sobald sich neue Physik zeigt, stellt sich die Aufgabe, ihre Beschaffenheit und das zugrunde liegende Modell zu finden. Erste Hinweise k\"onnen nat\"urlich schon das Massenspektrum und die Quantenzahlen wie z.B. die elektrische und die Farbladung der neuen Teilchen liefern. \\ In zwei sehr bekannten und gut untersuchten Modellklassen, Supersymmetrie und Extradimensionen, haben neue Teilchen allerdings sehr \"ahnliche Eigenschaften an der erreichbaren Energieskala. Beide Modelle f\"uhren Partnerteilchen zu den bekannten Standardmodell-Teilchen ein, die, abgesehen von der Masse, sehr \"ahnliche Eigenschaften besitzen. Aus diesem Grund ist es n\"otig, weitere Kriterien zu ihrer Unterscheidung einzusetzen.\\ Ein hilfreicher Unterschied ergibt sich aus der Konstruktion beider Modelle: W\"ahrend in Modellen mit Extradimensionen die Partnerteilchen gleichen Spin wie die Standardmodell-Teichen haben, ist der Spin der Partnerteilchen in supersymmetrischen Modellen um 1/2 verschieden. Dieser Unterschied hat nun interessante Auswirkungen auf die jeweilige Ph\"anomenologie der Modelle.\\ Zum Beispiel kann man ausnutzen, dass die unterschiedlichen Spins die absoluten Wirkungsquerschnitte beeinflussen. Diese Methode setzt allerdings voraus, dass man die Massen und Kopplungsst\"arken sehr genau kennt. Eine weitere Herangehensweise nutzt aus, dass Winkelverteilungen vom Spin der involvierten Teilchen abh\"angen k\"onnen. Eine wichtige darauf basierende Methode stellt einen Zusammenhang zwischen der invariante-Masse-Verteilung \$d\Gamma/d\sff\$ zweier Zerfallsprodukte und dem Spin des intermedi\"aren Teilchens, \"uber welches der Zerfall abl\"auft, her.\\ In dieser Arbeit untersuchen wir als erstes den Einfluss von Operatoren h\"oherer Ordnung auf die Spinbestimmung in Zerfallsketten. Wir klassifizieren als erstes die relevanten Operatoren der Dimension 5 und 6. Wir berechnen die neuen Beitr\"age und diskutieren ihre Auswirkungen auf die Bestimmung von Kopplungen und Spin in diesen Zerf\"allen.\\ Im weiteren betrachten wir zwei Szenarien, die nicht die \"ublichen Zerfallsketten beinhalten:\\ In Dreik\"orperzerf\"allen kann die oben erw\"ahnte Methode nicht angewendet werden, da das intermedi\"are Teilchen nicht auf die Massenschale gehen kann. Solche off-shell'' Zerf\"alle k\"onnen in Szenarien wie split-Supersymmetrie oder split-Universal Extra Dimensions'' wichtig sein. Man kann hier die sogenannte Narrow width approximation'' nicht anwenden, welche eine notwendige Voraussetzung f\"ur einen einfachen Zusammenhang zwischen Spin und der invariante-Masse-Verteilung ist. Wir arbeiten eine Strategie f\"ur diese Dreik\"orperzerf\"alle aus, mittels derer man zwischen den unterschiedlichen Spinszenarien unterscheiden kann. Diese Strategie beruht darauf, dass man hier die differentielle Zerfallsbreite als globalen Phasenraumfaktor mal einem Polynom in der invarianten Masse \$\sff\$ schreiben kann. Die hierbei auftretenden Koeffizienten sind nur Funktionen der involvierten Massen und Kopplungen, und wir zeigen, wie beispielsweise ihre Wertebereiche und Vorzeichen dazu benutzt werden k\"onnen, um den zugrunde liegenden Zerfall zu bestimmen. Am Ende testen wir diese Strategie in einer Reihe von Monte Carlo-Simulationen, und diskutieren auch den Einfluss des off-shell'' Teilchens. Im letzten Teil betrachten wir eine Topologie mit sehr kurzen Zefallsketten, in der man den oben genannten Zusammenhang zwischen Spin und invarianter Masse ebenfalls nicht anwenden kann. Wir untersuchen eine bestimmte Variable, die zur Unterscheidung von Supersymmetrie und Universal Extra Dimensions'' eingef\"uhrt wurde. Dabei nutzt man aus, dass sich das Problem im Hochenergielimes auf die zugrunde liegenden Produktionsprozesse reduziert. Wir diskutieren, wie man diese Variable auch in Szenarien anwenden kann, in denen dieser Limes keine gute N\"aherung darstellt. Dazu betrachten wir die m\"oglichen Spinszenarien mit renormierbaren Kopplungen und untersuchen im Detail, wie gut diese Variable zwischen verschiedenen Spin-, Massen- und Kopplungsszenarien unterscheiden kann. Wir finden beispielsweise, dass das Spinszenario, welches den supersymmetrischen Fall beinhaltet, von den meisten anderen Spinszenarien gut unterscheidbar ist.}, subject = {Elementarteilchenphysik}, language = {en} } @phdthesis{Weidinger2011, author = {Weidinger, Matthias}, title = {Variabilit{\"a}t entlang der Blazar-Sequenz - Hinweise auf die Zusammensetzung relativistischer Ausfl{\"u}sse Aktiver Galaxienkerne}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-70508}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2011}, abstract = {Die vorliegende Arbeit besch{\"a}ftigt sich mit der Abstrahlung von Aktiven Galaxienkernen. Das erste Maximum der charakteristischen Doppelpeakstruktur des \$\nu F_{\nu}\$-Spektrums vom Blazaren ist zweifelsfrei Synchrotronstrahlung hochenergetischer Elektronen innerhalb des relativistischen Ausflusses des zugrundeliegenden Aktiven Galaxienkerns. Die zum zweiten (hochenergetischen) Maximum beitragenden Strahlungsprozesse und Teilchenspezies hingegen sind Gegenstand aktueller Diskussionen. In dieser Arbeit wir ein vollst{\"a}ndig selbstkonsistentes und zeitabh{\"a}ngiges hybrides Emissionsmodell, welches auch Teilchenbeschleunigung ber{\"u}cksichtigt, entwickelt und auf verschiedene Blazar-Typen entlang der Blazar-Sequenz, von BL Lac Objekten mit verschiedenen Peakfrequenzen bis hin zu Flachspektrum-Radioquasaren, angewendet. Die spektrale Emission ersterer kann gut im rein leptonischen Grenzfall, d.h. der zweite \$\nu F_{\nu}\$-Peak kommt durch invers Compton-gestreute Synchrotronphotonen der abstrahlenden Elektronen selbst zustande, beschrieben werden. Zur Beschreibung letzterer muss man nicht-thermische Protonen innerhalb des Jets zulassen um die Dominanz des zweiten Maximums im Spektrum konsistent zu erkl{\"a}ren. In diesem Fall besteht der zweite Peak aus Protonensynchrotronstrahlung und Kaskadenstrahlung der photohadronischen Prozesse. Mit dem entwickelten Modell ist es m{\"o}glich auch die zeitliche Information, welche durch Ausbr{\"u}che von Blazaren bereitgestellt wird, auszunutzen um zum einen die freien Modellparameter weiter einzuschr{\"a}nken und -viel wichtiger- zum anderen leptonisch dominierte Blazare von hadronischen zu unterscheiden. Hierzu werden die typischen Zeitunterschiede in den Interbandlichtkurven als hadronischer Fingerabdruck benutzt.\\ Mit einer Stichprobe von 16 Spektren von zehn Blazaren entlang der Blazar-Sequenz, welche in unterschiedlichen Flusszust{\"a}nden und mit starker Variabilit{\"a}t beobachtet wurden, ist es m{\"o}glich die wichtigsten offenen Fragen der Physik relativistischer Ausbr{\"u}che in systematischer Art und Weise zu adressieren. Anhand der modellierten Ausbr{\"u}che kann man erkennen, dass sechs Quellen rein leptonisch dominiert sind, aber vier Protonen bis auf \$\gamma \approx 10^{11}\$ beschleunigen, was Auswirkungen auf die m{\"o}glichen Quellen extragalaktischer kosmischer Strahlung unter den Blazaren hat. Dar{\"u}ber hinaus findet sich eine Abh{\"a}ngigkeit zwischen dem Magnetfeld der Emissionsregion und der injizierten Leuchtkraft, welche unabh{\"a}ngig von den zugrunde liegenden Teilchenpopulationen G{\"u}ltigkeit besitzt. In diesem Zusammenhang l{\"a}sst sich die Blazar-Sequenz als ein evolution{\"a}res Szenario erkl{\"a}ren: die Sequenz \$FSRQ \rightarrow LBL/IBL \rightarrow HBL\$ kommt aufgrund abnehmender Gasdichte der Hostgalaxie und damit einhergehender abnehmender Akkretionsrate zustande, dies wird durch weitere kosmologische Beobachtungen best{\"a}tigt. Eine abnehmende Materiedichte innerhalb des relativistischen Ausflusses wird von einem abnehmenden Magnetfeld begleitet, d.h. aber auch, dass Protonen weit vor den Elektronen nicht mehr im Strahlungsgebiet gehalten werden k{\"o}nnen. Die Blazar-Sequenz ist also ein Maß f{\"u}r die Hadronizit{\"a}t des Jets. Dies erkl{\"a}rt zudem die Dichotomie von FSRQs und BL Lac Objekten sowie die Zweiteilung in anderen Erscheinungsformen von AGN, z.B. FR-I und FR-II Radiogalaxien.\\ W{\"a}hrend der Modellierung wird gezeigt, dass man Blazar-Spektren, speziell im hadronischen Fall, nicht mehr statisch betrachten kann, da es zu kumulierten Effekten aufgrund der langen Protonensynchrotronzeitskala kommt. Die niedrige Luminosit{\"a}t der Quellen und unterschiedlich lange Beobachtungszeiten verschiedener Experimente verlangen bei variablen Blazaren auch im leptonischen Fall eine zeitabh{\"a}ngige Betrachtung. Die Kurzzeitvariabilit{\"a}t scheint bei einzelnen Blazaren stets die selbe Ursache zu haben, unterscheidet sich aber bei der Betrachtung verschiedener Quellen. Zus{\"a}tzlich wird f{\"u}r jeden Blazar, der in verschiedenen Flusszust{\"a}nden beobachtet werden konnte, der Unterschied zwischen Lang- und Kurzzeitvariabilit{\"a}t, auch im Hinblick auf einen m{\"o}glichen globalen Grundzustand hin, betrachtet.}, subject = {Blazar}, language = {de} } @article{KaiserRiemerKnopf2011, author = {Kaiser, J. C. and Riemer, N. and Knopf, D. A.}, title = {Detailed heterogeneous oxidation of soot surfaces in a particle-resolved aerosol model}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-75440}, year = {2011}, abstract = {Using the particle-resolved aerosol model PartMC-MOSAIC, we simulate the heterogeneous oxidation of a monolayer of polycyclic aromatic hydrocarbons (PAHs) on soot particles in an urban atmosphere. We focus on the interaction of the major atmospheric oxidants (O3, NO2, OH, and NO3) with PAHs and include competitive co-adsorption of water vapour for a range of atmospheric conditions. For the first time detailed heterogeneous chemistry based on the P¨oschl-Rudich-Ammann (PRA) framework is modelled on soot particles with a realistic size distribution and a continuous range of chemical ages. We find PAH half-lives, 1/2, on the order of seconds during the night, when the PAHs are rapidly oxidised by the gas-surface reaction with NO3. During the day, 1/2 is on the order of minutes and determined mostly by the surface layer reaction of PAHs with adsorbed O3. Such short half-lives of surface-bound PAHs may lead to efficient conversion of hydrophobic soot into more hygroscopic particles, thus increasing the particles' aerosol-cloud interaction potential. Despite its high reactivity OH appears to have a negligible effect on PAH degradation which can be explained by its very low concentration in the atmosphere. An increase of relative humidity (RH) from 30\% to 80\% increases PAH half-lives by up to 50\%for daytime degradation and by up to 100\% or more for nighttime degradation. Uptake coefficients, averaged over the particle population, are found to be relatively constant over time for O3 (2×10-7 to 2×10-6) and NO2 (5×10-6 to 10-5) at the different levels of NOx emissions and RH considered in this study. In contrast, those for OH and NO3 depend strongly on the surface concentration of PAHs. We do not find a significant influence of heterogeneous reactions on soot particles on the gas phase composition. The derived half-lives of surfacebound PAHs and the time and particle population averaged uptake coefficients for O3 and NO2 presented in this paper can be used as parameterisations for the treatment of heterogeneous chemistry in large-scale atmospheric chemistry models.}, subject = {Physik}, language = {en} } @phdthesis{Wisniewski2011, author = {Wisniewski, Martina}, title = {Numerische Untersuchung von Turbulenz und Teilchentransport in der Heliosphaere}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-64652}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2011}, abstract = {Hochenergetische solare Teilchen werden bei ihrem Transport durch die Heliosph{\"a}re an turbulenten Magnetfeldern gestreut. F{\"u}r das Verst{\"a}ndnis dieses Streuprozesses ergeben sich aus heutiger Sicht zwei wesentliche Hindernisse: - Bei der Streuung hochenergetischer Teilchen an turbulenten Magnetfeldern handelt es sich um einen nichtlinearen Prozess, der durch analytische Theorien kaum zu beschreiben ist. - Der Streuprozess h{\"a}ngt stark von den tats{\"a}chlichen Magnetfeldern und somit auch von der Magnetfeldturbulenz ab. Unser bisheriges Verst{\"a}ndnis der heliosph{\"a}rischen Turbulenz ist leider aufgrund sp{\"a}rlicher experimenteller Daten deutlich eingeschr{\"a}nkt, was eine qualifizierte Umsetzung in analytischen und numerischen Ans{\"a}tzen deutlich erschwert. Dies machte in der Vergangenheit k{\"u}nstliche Annahmen f{\"u}r die Modellerstellung notwendig. In dieser Arbeit wird der Teilchentransport mit Hilfe der Simulation von Testteilchen in einem turbulenten, magnetohydrodynamischen Plasma untersucht. Durch die Testteilchen werden auch die nichtlinearen Streuprozesse korrekt wiedergegeben, wodurch das erste hier genannte Hindernis {\"u}berwunden wird. Dies wurde auch bereits in fr{\"u}heren numerischen Untersuchungen erfolgreich angewendet. Die Modellierung der Turbulenz f{\"u}r den Fall des Teilchentransports erfolgt in dieser Arbeit erstmalig auf Grundlage der magnetohydrodynamischen Gleichungen. Dabei handelt es sich um die mathematisch korrekte Wiedergabe der Magnetfeldturbulenz unterhalb der Ionen-Gyrofrequenz mit nur geringen numerischen Einschr{\"a}nkungen. Dar{\"u}ber hinaus erlaubt ein auf das physikalische Szenario anpassbarer Turbulenztreiber eine noch realistischere Simulation der Turbulenz. Durch diesen universell g{\"u}ltigen, numerischen Ansatz k{\"o}nnen f{\"u}r das zweite hier angegebene Hindernis jegliche k{\"u}nstlichen Annahmen vermieden werden. Die drei im Rahmen dieser Arbeit erstmals zusammengef{\"u}hrten Methoden (Testteilchen, magnetohydrodynamische Turbulenz, Turbulenztreiber) erm{\"o}glichen somit eine Untersuchung und Analyse von Transport- und Turbulenzph{\"a}nomenen mit herausragender Qualit{\"a}t, die insbesondere f{\"u}r den Fall des Teilchentransports einen direkten Anschluss an experimentelle Ergebnisse erm{\"o}glichen. Wichtige Ergebnisse im Rahmen dieser Arbeit sind: - der Nachweis der Drei-Wellen-Wechselwirkung f{\"u}r schwache und einsetzende starke Turbulenz. - eine Analyse der Anisotropie der Turbulenz im Bezug auf das Hintergrundmagnetfeld in Abh{\"a}ngigkeit vom Treibmodell. Insbesondere die Anisotropie ist experimentell bislang kaum erfassbar. - eine Untersuchung der Auswirkung der Gyroresonanzen auf die Diffusionskoeffizienten hochenergetischer solarer Teilchen in allgemeiner Form. - die Simulation des Teilchentransports in der Heliosph{\"a}re auf Grundlage experimenteller Messdaten. Die genauere Analyse der Simulationsergebnisse erm{\"o}glicht insgesamt einen Zugang zum Verst{\"a}ndnis des Transports, der durch experimentelle Untersuchungen nicht erfassbar ist. Bei der Simulation wurden lediglich die Magnetfeldst{\"a}rke sowie die untersuchte Teilchenenergie vorgegeben. Aus der Analyse der Simulationsergebnisse ergibt sich dieselbe mittlere freie Wegl{\"a}nge, wie sie auch durch andere Verfahren direkt aus den Messergebnissen gewonnen werden konnte. Auch die vorwiegende Ausrichtung der hochenergetischen Teilchen parallel und antiparallel zum Hintergrundmagnetfeld in der Simulation entspricht experimentellen Untersuchungen. Es zeigt sich, dass diese allein aus den resonanten Streuprozessen der Teilchen mit den Magnetfeldern resultiert. Des Weiteren werden die Art der Diffusion, der Energieverlust der Teilchen w{\"a}hrend des Transportprozesses sowie die G{\"u}ltigkeit der quasilinearen Theorie untersucht.}, subject = {Sonnenwind}, language = {de} } @phdthesis{Schenkel2011, author = {Schenkel, Alexander}, title = {Noncommutative Gravity and Quantum Field Theory on Noncommutative Curved Spacetimes}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-65823}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2011}, abstract = {{\"U}ber die letzten Jahrzehnte hat sich die nichtkommutative Geometrie zu einem etablierten Teilgebiet der reinen Mathematik und der theoretischen Physik entwickelt. Die Entdeckung, dass gewisse Grenzf{\"a}lle der Quantengravitation und Stringtheorie zu nichtkommutativer Geometrie f{\"u}hren, motivierte die Suche nach Physik jenseits des Standardmodells der Elementarteilchenphysik und der Einstein'schen allgemeinen Relativit{\"a}tstheorie im Rahmen von nichtkommutativen Geometrien. Einen ergiebigen Ansatz zu letzteren Theorien, welcher Deformationsquantisierung (Sternprodukte) mit Methoden aus der Theorie der Quantengruppen kombiniert, wurde von der Gruppe um Julius Wess entwickelt. Die resultierende Gravitationstheorie ist nicht nur imstande nichtkommutative Effekte der Raumzeit zu beschreiben, sondern sie erf{\"u}llt ebenfalls ein generalisiertes allgemeines Kovarianzprinzip, welches durch eine deformierte Hopf Algebra von Diffeomorphismen beschrieben wird. Gegenstand des ersten Teils dieser Dissertation ist es Symmetriereduktion im Rahmen von nichtkommutativer Gravitation zu verstehen und damit exakte L{\"o}sungen der nichtkommutativen Einstein'schen Gleichungen zu konstruieren. Diese Untersuchungen sind von großer Bedeutung um den physikalischen Inhalt dieser Theorien herauszuarbeiten und den Kontakt zu Anwendungen, z.B. im Rahmen nichtkommutativer Kosmologie und Physik schwarzer L{\"o}cher, herzustellen. Wir verallgemeinern die {\"u}bliche Methode der Symmetriereduktion, welche eine Standardtechnik im Auffinden von L{\"o}sungen der Einstein'schen Gleichungen ist, auf nichtkommutative Gravitation. Es wird gezeigt, dass unsere Methode zur nichtkommutativen Symmetriereduktion f{\"u}r ein gegebenes symmetrisches System zu bevorzugten Deformationen f{\"u}hrt. F{\"u}r Abelsche Drinfel'd Twists klassifizieren wir alle konsistenten Deformationen von r{\"a}umlich flachen Friedmann-Robertson-Walker Kosmologien und des Schwarzschild'schen schwarzen Loches. Aufgrund der deformierten Symmetriestruktur dieser Modelle k{\"o}nnen wir viele Beispiele von exakten L{\"o}sungen der nichtkommutativen Einstein'schen Gleichungen finden, bei welchen das nichtkommutative Metrikfeld mit dem klassischen {\"u}bereinstimmt. Im Fokus des zweiten Teils sind Quantenfeldtheorien auf nichtkommutativen gekr{\"u}mmten Raumzeiten. Dazu entwickeln wir einen neuen Formalismus, welcher algebraische Methoden der Quantenfeldtheorie mit nichtkommutativer Differentialgeometrie verkn{\"u}pft. Als Resultat unseres Ansatzes erhalten wir eine Observablenalgebra f{\"u}r skalare Quantenfeldtheorien auf einer großen Klasse von nichtkommutativen gekr{\"u}mmten Raumzeiten. Es wird eine pr{\"a}zise Relation zwischen dieser Algebra und der Observablenalgebra der undeformierten Quantenfeldtheorie hergeleitet. Wir studieren ebenfalls explizite Beispiele von deformierten Wellenoperatoren und finden, dass im Gegensatz zu dem einfachsten Modell des Moyal-Weyl deformierten Minkowski-Raumes, im Allgemeinen schon die Propagation freier Felder durch die nichtkommutative Geometrie beeinflusst wird. Die Effekte von konvergenten Deformationen werden in einfachen Spezialf{\"a}llen untersucht, und wir beobachten neue Aspekte in diesen Quantenfeldtheorien, welche sich in formalen Deformationen nicht zeigten. Zus{\"a}tzlich zu der erwarteten Nichtlokalit{\"a}t finden wir, dass sich die Beziehung zwischen der deformierten und der undeformierten Quantenfeldtheorie nichttrivial ver{\"a}ndert. Wir beweisen, dass dies zu einem verbesserten Verhalten der nichtkommutativen Theorie bei kurzen Abst{\"a}nden, d.h. im Ultravioletten, f{\"u}hrt. Im dritten Teil dieser Arbeit entwickeln wir Elemente eines leistungsf{\"a}higeren, jedoch abstrakteren, mathematischen Ansatzes zur Beschreibung der nichtkommutativen Gravitation. Das Hauptaugenmerk liegt auf globalen Aspekten von Homomorphismen zwischen und Zusammenh{\"a}ngen auf nichtkommutativen Vektorb{\"u}ndeln, welche fundamentale Objekte in der mathematischen Beschreibung von nichtkommutativer Gravitation sind. Wir beweisen, dass sich alle Homomorphismen und Zusammenh{\"a}nge der deformierten Theorie mittels eines Quantisierungsisomorphismus aus den undeformierten Homomorphismen und Zusammenh{\"a}ngen ableiten lassen. Es wird ebenfalls untersucht wie sich Homomorphismen und Zusammenh{\"a}nge auf Tensorprodukte von Moduln induzieren lassen. Das Verst{\"a}ndnis dieser Induktion erlaubt es uns die nichtkommutative Gravitationstheorie von Wess et al. um allgemeine Tensorfelder zu erweitern. Als eine nichttriviale Anwendung des neuen Formalismus erweitern wir unsere Studien zu exakten L{\"o}sungen der nichtkommutativen Einstein'schen Gleichungen auf allgemeinere Klassen von Deformationen.}, subject = {Nichtkommutative Geometrie}, language = {en} } @phdthesis{Adamek2011, author = {Adamek, Julian}, title = {Classical and Quantum Aspects of Anisotropic Cosmology}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-65908}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2011}, abstract = {The idea that our observable Universe may have originated from a quantum tunneling event out of an eternally inflating false vacuum state is a cornerstone of the multiverse paradigm. Modern theories that are considered as an approach towards the ultraviolet-complete fundamental theory of particles and gravity, such as the various types of string theory, even suggest that a vast landscape of different vacuum configurations exists, and that gravitational tunneling is an important mechanism with which the Universe can explore this landscape. The tunneling scenario also presents a unique framework to address the initial conditions of our observable Universe. In particular, it allows to introduce deviations from the cosmological concordance model in a controlled and well-motivated way. These deviations are a central topic of this work. An important feature in most of the theories mentioned above is the presumed existence of additional space dimensions in excess of the three which we observe in our every-day experience. It was realized that these extra dimensions could avoid our detection if they are compactified to microscopic length scales far beyond the reach of current experiments. There also seem to be natural mechanisms available for dynamical compactification in those theories. These typically lead to a vast landscape of different vacuum configurations which also may differ in the number of macroscopic dimensions, only the total number of dimensions being determined by the theory. Transitions between these vacuum configurations may hence open up new directions which were previously compact, spontaneously compactify some previously macroscopic directions, or otherwise re-arrange the configuration of compact and macroscopic dimensions in a more general way. From within the bubble Universe, such a process may be perceived as an anisotropic background spacetime - intuitively, the dimensions which open up may give rise to preferred directions. If our 3+1 dimensional observable Universe was born in a process as described above, one may expect to find traces of a preferred direction in cosmological observations. For instance, two directions could be curved like on a sphere, while the third space direction is flat. Using a scenario of gravitational tunneling to fix the initial conditions, I show how the primordial signatures in such an anisotropic Universe can be obtained in principle and work out a particular example in more detail. A small deviation from isotropy also has phenomenological consequences for the later evolution of the Universe. I discuss the most important effects and show that backreaction can be dynamically important. In particular, under certain conditions, a buildup of anisotropic stress in different components of the cosmic fluid can lead to a dynamical isotropization of the total stress-energy tensor. The mechanism is again demonstrated with the help of a physical example.}, subject = {Kosmologie}, language = {en} } @phdthesis{Englert2011, author = {Englert, Anja}, title = {Chaossynchronisation in Netzwerken mit zeitverz{\"o}gerten Kopplungen}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-65454}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2011}, abstract = {Die vorliegende Arbeit besch{\"a}ftigt sich mit der Chaossynchronisation in Netzwerken mit zeitverz{\"o}gerten Kopplungen. Ein Netzwerk chaotischer Einheiten kann isochron und vollst{\"a}ndig synchronisieren, auch wenn der Austausch der Signale einer oder mehreren Verz{\"o}gerungszeiten unterliegt. In einem Netzwerk identischer Einheiten hat sich als Stabilit{\"a}tsanalyse die Methode der Master Stability Funktion von Pecora und Carroll etabliert. Diese entspricht f{\"u}r ein Netzwerk gekoppelter iterativer Bernoulli-Abbildungen Polynomen vom Grade der gr{\"o}ßten Verz{\"o}gerungszeit. Das Stabilit{\"a}tsproblem reduziert sich somit auf die Untersuchung der Nullstellen dieser Polynome hinsichtlich ihrer Lage bez{\"u}glich des Einheitskreises. Eine solche Untersuchung kann beispielsweise numerisch mit dem Schur-Cohn-Theorem erfolgen, doch auch analytische Ergebnisse lassen sich erzielen. In der vorliegenden Arbeit werden Bernoulli-Netzwerke mit einer oder mehreren zeitverz{\"o}gerten Kopplungen und/oder R{\"u}ckkopplungen untersucht. Hierbei werden Aussagen {\"u}ber Teile des Stabilit{\"a}tsgebietes getroffen, welche unabh{\"a}ngig von den Verz{\"o}gerungszeiten sind. Des Weiteren werden Aussagen zu Systemen gemacht, welche sehr große Verz{\"o}gerungszeiten aufweisen. Insbesondere wird gezeigt, dass in einem Bernoulli-Netzwerk keine stabile Chaossynchronisation m{\"o}glich ist, wenn die vorhandene Verz{\"o}gerungszeit sehr viel gr{\"o}ßer ist als die Zeitskala der lokalen Dynamik, bzw. der Lyapunovzeit. Außerdem wird in bestimmten Systemen mit mehreren Verz{\"o}gerungszeiten anhand von Symmetriebetrachtungen stabile Chaossynchronisation ausgeschlossen, wenn die Verz{\"o}gerungszeiten in bestimmten Verh{\"a}ltnissen zueinander stehen. So ist in einem doppelt bidirektional gekoppeltem Paar ohne R{\"u}ckkopplung und mit zwei verschiedenen Verz{\"o}gerungszeiten stabile Chaossynchronisation nicht m{\"o}glich, wenn die Verz{\"o}gerungszeiten in einem Verh{\"a}ltnis von teilerfremden ungeraden ganzen Zahlen zueinander stehen. Es kann zudem Chaossynchronisation ausgeschlossen werden, wenn in einem bipartiten Netzwerk mit zwei großen Verz{\"o}gerungszeiten zwischen diesen eine kleine Differenz herrscht. Schließlich wird ein selbstkonsistentes Argument vorgestellt, das das Auftreten von Chaossynchronisation durch die Mischung der Signale der einzelnen Einheiten interpretiert und sich unter anderem auf die Teilerfremdheit der Zyklen eines Netzes st{\"u}tzt. Abschließend wird untersucht, ob einige der durch die Bernoulli-Netzwerke gefundenen Ergebnisse sich auf andere chaotische Netzwerke {\"u}bertragen lassen. Hervorzuheben ist die sehr gute {\"U}bereinstimmung der Ergebnisse eines Bernoulli-Netzwerkes mit den Ergebnissen eines gleichartigen Netzwerkes gekoppelter Halbleiterlasergleichungen, sowie die {\"U}bereinstimmungen mit experimentellen Ergebnissen eines Systems von Halbleiterlasern.}, subject = {Chaos}, language = {de} } @phdthesis{Rueger2011, author = {R{\"u}ger, Michael}, title = {Ein zeitabh{\"a}ngiges, selbstkonsistentes hadronisch-leptonisches Strahlungsmodell zur Modellierung der Multiwellenl{\"a}ngenemission von Blazaren}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-56955}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2011}, abstract = {Diese Arbeit besch{\"a}ftigt sich mit Strahlungsprozessen in Blazaren. Bei den Blazaren handelt es sich um eine Unterkategorie der aktiven Galaxienkerne, bei denen die Jetachse in Richtung des Beobachters zeigt. Charakteristisch f{\"u}r die Blazare ist ein Multifrequenzspektrum der Photonen, welches sich vom Radiobereich bis hin zur Gamma-Strahlung mit TeV-Energien erstreckt. Insbesondere der Gamma-Bereich r{\"u}ckt aktuell in den Fokus der Betrachtung mit Experimenten wie zum Beispiel FERMI und MAGIC. Ziel dieser Arbeit ist die Modellierung der auftretenden Strahlungsprozesse und die Beschreibung der Multifrequenzspektren der Blazare mit Hilfe eines hadronisch-leptonischen Modells. Grundlage hierf{\"u}r ist ein selbstkonsistentes Synchrotron-Selbst-Compton-Modell (SSC), welches zur Beschreibung des Spektrums der Quelle 1 ES 1218+30.4 verwendet wird. Dabei wird die Parameterwahl unterst{\"u}tzt durch eine Absch{\"a}tzung der Masse des zentralen schwarzen Loches. Das hier behandelte SSC-Modell wird dahingehend untersucht, wie es sich unter Ver{\"a}nderung der Modellparameter verh{\"a}lt. Dabei werden Abh{\"a}ngigkeiten des Photonenspektrums von {\"A}nderungsfaktoren der Parameter abgeleitet. Außerdem werden diese Abh{\"a}ngigkeiten in Relation gesetzt und aus dieser Betrachtung ergibt sich die Schlussfolgerung, dass unter der Voraussetzung eines festen Spektralindex der Elektronenverteilung die Wahl eines Parametersatzes zur Modellierung eines Photonenspektrums eindeutig ist. Zur Einf{\"u}hrung eines zeitabh{\"a}ngigen, hadronischen Modells wird das SSCModell um die Anwesenheit nichtthermischer Protonen erweitert. Dadurch kann Proton-Synchrotron-Strahlung einen Beitrag im Gamma-Bereich leisten. Außerdem werden durch Proton-Photon-Wechselwirkung Pionen erzeugt. Aus deren Zerfall werden zusammen mit der Paarbildung aus Photon-Photon-Absorption sekund{\"a}re Elektronen und Positronen produziert, die wiederum zum Hochenergiespektrum beitragen. Neben den Pionen werden bei der Proton-Photon- Wechselwirkung außerdem noch Neutrinos und Neutronen erzeugt, die einen direkten Einblick in die Emissionsregion erlauben. Das hier vorgestellte hadronische Modell wird auf die Quelle 3C 279 angewandt. F{\"u}r diese Quelle reicht mit der Detektion im VHE-Bereich der SSCAnsatz nicht aus, um das Photonenspektrum zu beschreiben. Mit dem vorgelegten Modell gelingt die Beschreibung des Spektrums in den SSC-kritischen Bereichen sehr gut. Insbesondere k{\"o}nnen verschiedene Flusszust{\"a}nde modelliert und allein durch Ver{\"a}nderung der Maximalenergien von Protonen und Elektronen ineinander {\"u}berf{\"u}hrt werden. Diese einfache M{\"o}glichkeit der Modellierung der Variabilit{\"a}t der Quelle unterstreicht die Wahl des hadronischen Ansatzes. Somit wird hier ein sehr gutes Werkzeug zur Untersuchung der Emissionsprozesse in Blazaren geliefert. Dar{\"u}ber hinaus ist mit der Absch{\"a}tzung des Neutrino-Flusses zwar die Detektion von 3C 279 als Punktquelle mit IceCube unwahrscheinlich, jedoch liefert das Modell generell die M{\"o}glichkeit im Kontext des Multimessenger-Ansatzes Antworten zu liefern. Im gleichen Kontext wird auch der Beitrag zur kosmischen Strahlung durch entweichende Neutronen untersucht.}, subject = {Blazar}, language = {de} } @phdthesis{Tang2011, author = {Tang, Jian}, title = {Phenomenology of Neutrino Oscillations at the Neutrino Factory}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-66765}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2011}, abstract = {We consider the prospects for a neutrino factory measuring mixing angles, the CP violating phase and mass-squared differences by detecting wrong-charge muons arising from the chain \$\mu^+\to\nu_e\to\nu_\mu\to\mu^-\$ and the right-charge muons coming from the chain \$\mu^+\to\bar{\nu}_\mu\to\bar{\nu}_\mu\to\mu^+\$ (similar to \$\mu^-\$ chains), where \$\nu_e\to\nu_\mu\$ and \$\bar{\nu}_\mu\to\bar{\nu}_\mu\$ are neutrino oscillation channels through a long baseline. First, we study physics with near detectors and consider the treatment of systematic errors including cross section errors, flux errors, and background uncertainties. We illustrate for which measurements near detectors are required, discuss how many are needed, and what the role of the flux monitoring is. We demonstrate that near detectors are mandatory for the leading atmospheric parameter measurements if the neutrino factory has only one baseline, whereas systematic errors partially cancel if the neutrino factory complex includes the magic baseline. Second, we perform the baseline and energy optimization of the neutrino factory including the latest simulation results from the magnetized iron neutrino detector (MIND). We also consider the impact of \$\tau\$ decays, generated by appearance channels \$\nu_\mu \rightarrow \nu_\tau\$ and \$\nu_e \rightarrow \nu_\tau\$, on the discovery reaches of the mass orderings, the leptonic CP violation, and the non-zero \$\theta_{13}\$, which we find to be negligible for the considered detector. Third, we make a comparison of a high energy neutrino factory to a low energy neutrino factory and find that they are just two versions of the same experiment optimized for different regions of the parameter space. In addition, we briefly comment on whether it is useful to build the bi-magic baseline at the low energy neutrino factory. Finally, the effects of one additional massive sterile neutrino are discussed in the context of a combined short and long baseline setup. It is found that near detectors can provide the required sensitivity at the LSND-motivated \$\Delta m_{41}^2\$-range, while some sensitivity can also be obtained in the region of the atmospheric mass splitting introduced by the sterile neutrino from the long baselines.}, subject = {Neutrinooszillation}, language = {en} } @phdthesis{Simon2011, author = {Simon, Dennis}, title = {Aspects in the fate of primordial vacuum bubbles}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-67019}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2011}, abstract = {At the present day the idea of cosmological inflation constitutes an important extension of Big Bang theory. Since its appearance in the early 1980's many physical mechanisms have been worked out that put the inflationary expansion of space that proceeds the Hot Big Bang on a sound theoretical basis. Among the achievements of the theory of inflation are the explanaition of the almost Euclidean geometry of 'visible'space, the homogeneity of the cosmic background radiation but, in particular, also the tiny inhomogeneity of a relative amplitude of 10-5. In many models of inflation the inflationary phase ends only locally. Hence, there exists the possibility that the inflationary process still goes on in regions beyond our visual horizon. This property is commonly termed 'eternal inflation'. In the framework of a cosmological scalar fields, eternal inflation can manifest itself in a variety of ways. On the one hand fluctuations of the field, if sufficiently large, can work against the classical trajectory and therefore counteract the end of inflation. In regions where this is the case the accelerated expansion of space continues at a higher rate. In parts of this region the process may replicate itself again and in this way may continue throughout all of time. Space and field are said to reproduce themselves. On the other hand, a mechanism that can occur in addition or independent of the latter, is so called vacuum tunneling. If the potential of the scalar field has several local minima, a semi-classical calculation suggests that within a spherical region, a bubble, the field can tunnel to another state. The respective tunneling rates depend on the potential difference and the shape of the potential between the states. Generally, the tunneling rate is exponentially suppressed, which means that the inflation lasts for a long time before tunneling takes place. The ongoing inflationary process effectively reduces local curvature, anistotropy and inhomogeneity, so that this property is known as the 'cosmic no-hair conjecture'. For this reason cosmological considerations of the evolution of bubbles thus far almost entirely involved vacuum (de Sitter) backgrounds. However, new insights in the framework of string theory suggest high tunneling rates which allow for the possibility of bubble nucleation in non-vacuum dominated backgrounds. In this case the evolution of the bubble depends on the properties of the background spacetime. A deeper introduction in chapter 4 is followed by the presentation of the Lema{\^i}tre-Tolman spacetime in chapter 5 which constitutes the background spacetime in the study of the effect of matter and inhomogeneity on the evolution of vacuum bubbles. In chapter 6 we explicitly describe the application of the 'thin-shell' formalism and the resulting system of equations. This is succeeded in chapter 7 by the detailed analysis of bubble evolution in various limits of the Lema{\^i}tre-Tolman spacetime and a Robertson-Walker spacetime with a rapid phase transition. The central observations are that the presence of dust, at a fixed surface energy density, goes along with a smaller nucleation volume and possibly leads to a a collapse of the bubble. In an expanding background, the radially inhomogeneous dust profile is efficiently diluted so that there is essentially no effect on the evolution of the domain wall. This changes in a radially inhomogeneous curvature profile, positive curvature decelerates the expansion of the bubble. Moreover, we point out that the adopted approach does not allow for a treatment of a, physically expected, matter transfer so that the results are to be understood as preliminary under this caveat. In the second part of this thesis we consider potential observable consequences of bubble collisions in the cosmic microwave background radiation. The topological nature of the signal suggests the use of statistics that are well suited to quantify the morphological properties of the temperature fluctuations. In chapter 10 we present Minkowski Functionals (MFs) that exactly provide such statistics. The presented error analysis allows for a higher precision of numerical MFs in comparison to earlier methods. In chapter 12 we present the application of our algorithm to a Gaussian and a collision map. We motivate the expected MFs and extract their numerical counterparts. We find that our least-squares fitting procedure accurately reproduces an underlying signal only when a large number of realizations of maps are averaged over, while for a single WMAP and PLANCK resolution map, only when a highly prominent disk, with |δT| = 2√σG and ϑd = 40◦, we are able to recover the result. This is unfortunate, as it means that MF are intrinsically too noisy to be able to distinguish cold and hot spots in the CMB for small sizes.}, subject = {Kosmologie}, language = {en} } @phdthesis{Liebler2011, author = {Liebler, Stefan}, title = {LHC phenomenology and higher order electroweak corrections in supersymmetric models with and without R-parity}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-69367}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2011}, abstract = {During the last decades the standard model of particle physics has evolved to one of the most precise theories in physics, describing the properties and interactions of fundamental particles in various experiments with a high accuracy. However it lacks on some shortcomings from experimental as well as from theoretical point of view: There is no approved mechanism for the generation of masses of the fundamental particles, in particular also not for the light, but massive neutrinos. In addition the standard model does not provide an explanation for the observance of dark matter in the universe. Moreover the gauge couplings of the three forces in the standard model do not unify, implying that a fundamental theory combining all forces can not be formulated. Within this thesis we address supersymmetric models as answers to these various questions, but instead of focusing on the most simple supersymmetrization of the standard model, we consider basic extensions, namely the next-to-minimal supersymmetric standard model (NMSSM), which contains an additional singlet field, and R-parity violating models. R-parity is a discrete symmetry introduced to guarantee the stability of the proton. Using lepton number violating terms in the context of bilinear R-parity violation and the munuSSM we are able to explain neutrino physics intrinsically supersymmetric, since those terms induce a mixing between the neutralinos and the neutrinos. Since 2009 the Large Hadron Collider (LHC) at CERN explores the new energy regime of Tera-electronvolt, allowing the production of potentially existing heavy particles by the collision of protons. Thus the near future might provide answers to the open questions of mass generation in the standard model and show hints towards physics beyond the standard model. Therefore this thesis works out the phenomenology of the supersymmetric models under consideration and tries to point out differences to the well-known features of the simplest supersymmetric realization of the standard model. In case of the R-parity violating models the decays of the light neutralinos can result in displaced vertices. In combination with a light singlet state these displaced vertices might offer a rich phenomenology like non-standard Higgs decays into a pair of singlinos decaying with displaced vertices. Within this thesis we present some calculations at next order of perturbation theory, since one-loop corrections provide possibly large contributions to the tree-level masses and decay widths. We are using an on-shell renormalization scheme to calculate the masses of neutralinos and charginos including the neutrinos and leptons in case of the R-parity violating models at one-loop level. The discussion shows the similarities and differences to existing calculations in another renormalization scheme, namely the DRbar scheme. Moreover we consider two-body decays of the form chi_j^0 -> chi_l^\pm W^\mp involving a heavy gauge boson in the final state at one-loop level. Corrections are found to be large in case of small or vanishing tree-level decay widths and also for the R-parity violating decay of the lightest neutralino chi_1^0 -> l^\pm W^\mp. An interesting feature of the models based on bilinear R-parity violation is the correlation between the branching ratios of the lightest neutralino decays and the neutrino mixing angles. We discuss these relations at tree-level and for two-body decays chi_1^0 -> l^\pm W^\mp also at one-loop level, since only the full one-loop corrections result in the tree-level expected behavior. The appendix describes the two programs MaCoR and CNNDecays being developed for the analysis carried out in this thesis. MaCoR allows for the calculation of mass matrices and couplings in the models under consideration and CNNDecays is used for the one-loop calculations of neutralino and chargino mass matrices and the two-body decay widths.}, subject = {Supersymmetrie}, language = {en} } @phdthesis{Elsaesser2011, author = {Els{\"a}sser, Dominik Martin}, title = {Indirect Search for Dark Matter in the Universe - the Multiwavelength and Multiobject Approach}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-69464}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2011}, abstract = {Dunkle Materie ist ein zentraler Bestandteil der modernen Kosmologie, und damit von entscheidender Bedeutung f{\"u}r unser Verst{\"a}ndnis der Strukturbildung im Universum. Das offensichtliche Fehlen von elektromagnetischer Wechselwirkung in Kombination mit unabh{\"a}ngigen Messungen der Energiedichte der baryonischen Materie {\"u}ber die H{\"a}ufigkeit der primordialen leichten Elemente weisen auf eine nicht-baryonische Natur der Dunklen Materie hin. Die Wirkung der Dunklen Materie bei der Strukturbildung zeigt weiterhin dass ihre Konstituenten kalt sind, also zum Zeitpunkt des Gleichgewichts zwischen Strahlung und Materie eine Temperatur kleine als ihre Ruhemasse aufwiesen. Generische Kandidaten f{\"u}r das Dunkelmaterie-Teilchen sind stabile, schwach wechselwirkende Teilchen mit Ruhemassen von der Gr{\"o}ßenordnung der Skala der elektroschwachen Symmetriebrechung, wie sie zum Beispiel in der Supersymmetrie bei erhaltener R-Parit{\"a}t vorkommen. Derartige Teilchen frieren auf nat{\"u}rliche Weise im fr{\"u}hen Universum mit kosmologisch relevanten Reliktdichten aus. Die fortschreitende Strukturbildung im Universum f{\"u}hrt dann zur Bildung von {\"u}berdichten Regionen, in denen die Dunkelmaterie-Teilchen wiederum in signifikantem Ausmaß annihilieren k{\"o}nnen. Dadurch w{\"u}rde ein potentiell detektierbarer Fluß von Hochenergie-Teilchen einschließlich Photonen aus den instabilen Zwischenprodukten der Annihilationsereignisse erzeugt. Die Spektren dieser Teilchen w{\"u}rden R{\"u}ckschl{\"u}sse auf die Masse und den Annihilations-Querschnitt als wichtige Gr{\"o}ßen zur mikrophysikalischen Identifikation der Dunkelmaterie-Teilchen erlauben. Darin liegt die zentrale Motivation f{\"u}r indirekte Suchen nach der Dunklen Materie. Zum gegenw{\"a}rtigen Zeitpunkt jedoch haben weder diese indirekten Suchen, noch direkte Methoden zur Suche nach elastischen Streuereignissen zwischen Dunkelmaterie-Teilchen und Atomkernen sowie Beschleunigerexperimente einen eindeutigen Nachweis von Dunkelmaterie-Teilchen erbracht. Das an sich stellt keine {\"U}berraschung dar, denn die zu erwartenden Signale sind aufgrund der schwachen Wechselwirkung der Teilchen nur von geringer Intensit{\"a}t. Im Falle der indirekten Suchen steht zu erwarten, dass selbst f{\"u}r die gr{\"o}ßten Massekonzentrationen im Universum die St{\"a}rke des Annihilationssignals der Dunklen Materie den durch astrophysikalische Quellen verursachten Untergrund nicht {\"u}berschreitet. Die M{\"o}glichkeit der sicheren Unterscheidung zwischen einem m{\"o}glichen Signal aus der Annihilation der Dunklen Materie und eben diesem Untergrund ist daher entscheidend f{\"u}r die Erfolgsaussichten der indirekten Suchen. In der vorliegenden Arbeit wird eine neuartige Suchstrategie ausgearbeitet und vorgestellt, deren zentrale Komponente die Auswahl von Beobachtungszielen aus einem breiten Massebereich, die Kontrolle der astrophysikalischen Untergr{\"u}nde, und die Einbeziehung von Daten aus mehreren Wellenl{\"a}ngenbereichen ist. Die durchgef{\"u}hrten Beobachtungen werden vorgestellt und interpretiert. Ein Ergebnis ist, dass die Unsicherheiten in Bezug auf die Verteilung der Dunklen Materie in Halos und deren individuelle Dichtestruktur, sowie in Bezug auf die m{\"o}gliche Verst{\"a}rkung des Annihilationssignales durch Substruktur, im Falle der massearmen Halos (wie zum Beispiel bei den Zwerggalaxien) gr{\"o}ßer ist als bei massereichen Halos, wie denen der Galaxienhaufen. Andererseits weisen die massereichen Halos gr{\"o}ßere Unsicherheiten in Hinblick auf die zu erwartenden rein astrophysikalischen Untergr{\"u}nde auf. Die Unsicherheiten in Bezug auf die bisher unbekannte Teilchenphysik jenseits des Standardmodells schließlich sind unabh{\"a}ngig von der Masse der beobachteten Halos. Im Zusammenspiel erm{\"o}glichen es diese unterschiedlichen Skalierungsverhalten, die globale Unsicherheit durch eine kombinierte Analyse der Beobachtungen von Halos mit verschiedenen Massen, die einen bedeutenden Teil der Masseskala abdecken, nennenswert zu reduzieren. Diese Strategie wurde im Rahmen des wissenschaftlichen Beobachtungsprogrammes des MAGIC Teleskopsystems implementiert. Es wurden Beobachtungen von Zwerggalaxien sowie des Virgo- und des Perseus-Galaxienhaufens durchgef{\"u}hrt. Die resultierenden Grenzen auf Gammastrahlung aus der Annihilation von schwach wechselwirkenden, massereichen Teilchen geh{\"o}ren zum Zeitpunkt dieser Niederschrift zu den st{\"a}rksten Grenzen aus indirekten Suchen nach der Dunklen Materie. Die so gewonnenen Grenzen auf die Annihilations-Fl{\"u}sse schr{\"a}nken einige in der Literatur diskutierte und durch aussergew{\"o}hnlich große Annihilations-Fl{\"u}sse gekennzeichnete Szenarien stark ein.}, subject = {Gammastrahlung}, language = {en} } @phdthesis{Ganse2012, author = {Ganse, Urs}, title = {Kinetische Simulationen solarer Typ II Radiobursts}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-73676}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2012}, abstract = {Die Emission solarer Typ II Radiobursts ist ein seit Jahrzehnten beobachtetes Ph{\"a}nomen der heliosph{\"a}rischen Plasmaphysik. Diese Radiobursts, die im Zusammenhang mit der Propagation koronaler Schockfronten auftreten, zeigen ein charakteristisches, zweibandiges Emissionsspektrum. Mit expandierendem Schock driften sie zu niedrigeren Frequenzen. Analytische Theorien dieser Emission sagen nichtlineare Plasmawellenwechselwirkung als Ursache voraus, doch aufgrund des geringen Sonnenabstands der Emissionsregion ist die in-situ Datenlage durch Satellitenmessungen {\"a}usserst schlecht, so dass eine endg{\"u}ltige Verifikation der vorhergesagten Vorg{\"a}nge bisher nicht m{\"o}glich war. Mit Hilfe eines kinetischen Plasma-Simulationscodes nach dem Particle-in-Cell Prinzip wurde in dieser Dissertation die Plasmaumgebung in der Foreshock-Region einer koronalen Schockfront modelliert. Das Propagations- und Kopplungsverhalten elektrostatischer und elektromagnetischer Wellenmoden wurde untersucht. Die vollst{\"a}ndige r{\"a}umliche Information {\"u}ber die Wellenzusammensetzung in der Simulation erlaubt es, die Kinematik nichtlinearer Wellenkopplungen genauestens zu untersuchen. Es zeigte sich ein mit der analytischen Theorie der Drei-Wellen-Wechselwirkung konsistentes Bild der Erzeugung solarer Radiobursts: durch elektromagnetischen Zerfall elektrostatischer Moden kommt es zur Erzeugung fundamentaler, sowie durch Verschmelzung gegenpropagierender elektrostatischer Moden zur Anregung harmonischer Radioemission. Kopplungsst{\"a}rken und Winkelabh{\"a}ngigkeit dieser Prozesse wurden untersucht. Mit dem somit zur Verf{\"u}gung stehenden, numerischen Laborsystem wurde die Parameter-Abh{\"a}ngigkeit der Wellenkopplungen und entstehenden Radioemissionen bez{\"u}glich St{\"a}rke des Elektronenbeams und des solaren Abstandes untersucht.}, subject = {Heliosph{\"a}re}, language = {de} } @phdthesis{Lange2012, author = {Lange, Sebastian}, title = {Turbulenz und Teilchentransport in der Heliosph{\"a}re - Simulationen von inkompressiblen MHD-Plasmen und Testteilchen -}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-74012}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2012}, abstract = {Die Herkunft hochenergetischer solarer Teilchen konnte in den vergangenen Jahren eindeutig auf Schockbeschleunigung an koronalen Masseausw{\"u}rfen zur{\"u}ckgef{\"u}hrt werden. Durch resonante Interaktionen zwischen Wellen und Teilchen werden zum einen geladene Teilchen unter Ver{\"a}nderung ihrer Energie gestreut, zum anderen wird die Dynamik der Plasmawellen in solchen Beschleunigungsregionen durch diese Prozesse von selbstgenerierten Wellenmoden maßgeblich beeinflusst. Mittels numerischer Modellierungen wurden im Rahmen dieser Arbeit die grundlegenden physikalischen Regimes der Turbulenz und des Teilchentransports beschrieben. Die Simulation der Plasmadynamik bedient sich der Methodik der Magnetohydrodynamik, wohingegen kinetische Einzelteilchen durch die elementaren Bewegungsgleichungen der Elektrodynamik berechnet werden. Es konnten die Turbulenztheorien von Goldreich und Sridhar unter heliosph{\"a}rischen Bedingungen bei drei solaren Radien best{\"a}tigt werden. Vor allem zeigten sich Hinweise f{\"u}r das Erreichen der kritischen Balance, einem Schl{\"u}sselparameter dieser Theorien. Weiterhin werden Ergebnisse der dynamischen Entwicklung angeregter Wellenmoden pr{\"a}sentiert, in denen die Bedeutsamkeit f{\"u}r die gesamte Turbulenz gezeigt werden konnte. Als zentraler Prozess bei hohen Energien hat sich das wave-steepening herausgestellt, das als effizienter Energietransportmechanismus in paralleler Richtung zum Hintergrundmagnetfeld identifiziert wurde und somit turbulente Strukturen bei hohen parallelen Wellenzahlen erkl{\"a}rt, deren Entstehung das Goldreich-Sridhar Modell nicht beschreiben kann. Dar{\"u}ber hinaus wurden grundlegende Erkenntnisse {\"u}ber die quasilineare Theorie des Teilchentransports erzielt. Im Speziellen konnte ein tieferes Verst{\"a}ndnis f{\"u}r die Interpretation der Diffusionskoeffizienten von Welle-Teilchen Wechselwirkungen erlangt werden. Simulationen zur Streuung an angeregten Wellenmoden zeigten erstmals komplexe resonante Strukturen die im Rahmen analytischer Modelle nicht mehr ad{\"a}quat beschrieben werden k{\"o}nnen.}, subject = {Heliosph{\"a}re}, language = {de} } @phdthesis{Uhlemann2012, author = {Uhlemann, Christoph Frank}, title = {Holographic Description of Curved-Space Quantum Field Theory and Gravity}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-74362}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2012}, abstract = {The celebrated AdS/CFT dualities provide a window to strongly-coupled quantum field theories (QFTs), which are realized in nature at the most fundamental level on the one hand, but are hardly accessible for the standard mathematical tools on the other hand. The prototype examples of AdS/CFT relate classical supergravity theories on (d+1)-dimensional anti-de Sitter space (AdS) to strongly-coupled d-dimensional conformal field theories (CFTs). The AdS spacetimes admit a timelike conformal boundary, on which the dual CFT is defined. In that sense the AdS/CFT dualities are holographic, and this new approach has led to remarkable progress in understanding strongly-coupled QFTs defined on Minkowski space and on the Einstein cylinder. On the other hand, the study of QFT on more generic curved spacetimes is of fundamental interest and non-trivial already for free theories. Moreover, understanding the properties of gravity as a quantum theory remains among the hardest problems to solve in physics. Both of these issues can be studied holographically and we investigate here generalizations of AdS/CFT involving on the lower-dimensional side QFTs on curved backgrounds and as a further generalization gravity. In the first part we expand on the holographic description of QFT on fixed curved backgrounds, which involves gravity on an asymptotically-AdS space with that prescribed boundary structure. We discuss geometries with de Sitter and AdS as conformal boundary to holographically describe CFTs on these spacetimes. After setting up the procedure of holographic renormalization we study the reflection of CFT unitarity properties in the dual bulk description. The geometry with AdS on the boundary exhibits a number of interesting features, mainly due to the fact that the boundary itself has a boundary. We study both cases and resolve potential tensions between the unitarity properties of the bulk and boundary theories, which would be incompatible with a duality. The origin of these tensions is partly in the structure of the geometry with AdS conformal boundary, while another one arises for a particular limiting case where the bulk and boundary descriptions naively disagree. Besides technical challenges, the hierarchy of boundaries for the geometry with AdS conformal boundary offers an interesting option. Namely, having the dual theory on the conformal boundary itself defined on an AdS space offers the logical possibility of implementing a second instance of AdS/CFT. We discuss an appropriate geometric setting allowing for the notion of the boundary of a boundary and identify limitations for such multi-layered dualities. In the second part we consider five-dimensional supergravities whose solutions can be lifted to actual string-theory backgrounds. We work out the asymptotic structure of the theories on asymptotically-AdS spaces and calculate the Weyl anomaly of the dual CFTs. These holographic calculations confirm the expectations from the field-theory side and provide a non-trivial test of the AdS/CFT conjecture. Moreover, building on the previous results we show that in addition to the usual Dirichlet also more general boundary conditions can be imposed. That allows to promote the boundary metric to a dynamical quantity and is expected to yield a holographic description for a conformal supergravity on the boundary. The boundary theory obtained this way exhibits pathologies such as perturbative ghosts, which is in fact expected for a conformal gravity. The fate of these ghosts beyond perturbation theory is an open question and our setting provides a starting point to study it from the string-theory perspective. That discussion leads to a regime where the holographic description of the boundary theory requires quantization of the bulk supergravity. A necessary ingredient of any supergravity is a number of gravitinos as superpartners of the graviton, for which we thus need an effective-QFT description to make sense of AdS/CFT beyond the limit where bulk theory becomes classical. In particular, quantization should be possible not only on rigid AdS, but also on generic asymptotically-AdS spacetimes which may not be Einstein. In the third part we study the quantization and causality properties of the gravitino on Friedmann-Robertson-Walker spacetimes to explicitly show that a consistent quantization can be carried out also on non-Einstein spaces, in contrast to claims in the recent literature. Furthermore, this reveals interesting non-standard effects for the gravitino propagation, which in certain cases is restricted to regions more narrow than the expected light cones.}, subject = {AdS-CFT-Korrespondenz}, language = {en} } @phdthesis{Schelter2012, author = {Schelter, J{\"o}rg}, title = {The Aharonov-Bohm effect and resonant scattering in graphene}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-74662}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2012}, abstract = {In this thesis, the electronic transport properties of mesoscopic condensed matter systems based on graphene are investigated by means of numerical as well as analytical methods. In particular, it is analyzed how the concepts of quantum interference and disorder, which are essential to mesoscopic devices in general, are affected by the unique electronic and transport properties of the graphene material system. We consider the famous Aharonov-Bohm effect in ring-shaped transport geometries, and, besides providing an overview over the recent developments on the subject, we study the signatures of fundamental phenomena such as Klein tunneling and specular Andreev reflection, which are specific to graphene, in the magnetoconductance oscillations. To this end, we introduce and utilize a variant of the well-known recursive Green's function technique, which is an efficient numerical method for the calculation of transport observables in effectively non-interacting open quantum systems in the framework of a tight binding model. This technique is also applied to study the effects of a specific kind of disorder, namely short-range resonant scatterers, such as strongly bound adatoms or molecules, that can be modeled as vacancies in the graphene lattice. This numerical analysis of the conductance in the presence of resonant scatterers in graphene leads to a non-trivial classification of impurity sites in the graphene lattice and is further substantiated by an independent analytical treatment in the framework of the Dirac equation. The present thesis further contains a formal introduction to the topic of non-equilibrium quantum transport as appropriate for the development of the numerical technique mentioned above, a general introduction to the physics of graphene with a focus on the particular phenomena investigated in this work, and a conclusion where the obtained results are summarized and open questions as well as potential future developments are highlighted.}, subject = {Graphen}, language = {en} } @misc{ReentsSchiekel2012, author = {Reents, Georg and Schiekel, Bernhard}, title = {In memoriam Karl Kraus}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-71296}, year = {2012}, abstract = {Prof. Dr. Karl Kraus Forscher und Lehrer am Physikalischen Institut der Universit{\"a}t W{\"u}rzburg Curriculum vitae und Publikationsliste}, subject = {Kraus}, language = {de} } @phdthesis{Kiesel2012, author = {Kiesel, Maximilian Ludwig}, title = {Unconventional Superconductivity in Cuprates, Cobaltates and Graphene: What is Universal and what is Material-Dependent in strongly versus weakly Correlated Materials?}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-76421}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2012}, abstract = {Eine allgemeing{\"u}ltige Theorie f{\"u}r alle unterschiedlichen Arten von unkonventionellen Supraleitern ist immer noch eine der ungel{\"o}sten Kernfragen der Festk{\"o}rperphysik. Momentan ist es nicht einmal bewiesen, dass es {\"u}berhaupt einen gemeinsamen grundlegenden Mechanismus gibt, sondern es m{\"u}ssen vielleicht mehrere verschiedene Ursachen f{\"u}r unkonventionelle Supraleitung ber{\"u}cksichtigt werden. Der Einfluss der Elektron-Phonon-Wechselwirkung ist dabei noch nicht abschließend gekl{\"a}rt. In dieser Dissertation wird ein rein elektronischer Paarungsmechanismus untersucht, in welchem die Paarung durch Spin-Fluktuationen vermittelt wird, was nach dem aktuellen Stand der Forschung auf dem Gebiet der unkonventionellen Supraleiter am wahrscheinlichsten ist. Der Schwerpunkt liegt dabei auf der Bestimmung von Material-unabh{\"a}ngigen Eigenschaften der supraleitenden Phase. Diese k{\"o}nnen durch eine Auswahl sehr unterschiedlicher Systeme herausgearbeitet werden. Eine Untersuchung der Phasendiagramme gibt außerdem Auskunft dar{\"u}ber, welche konkurrierenden Quantenfluktuationen den supraleitenden Zustand abschw{\"a}chen oder verst{\"a}rken. F{\"u}r diese Analyse von sehr unterschiedlichen supraleitenden Materialien ist der Einsatz einer einzelnen numerischen L{\"o}sungsmethode unzureichend. F{\"u}r diese Dissertation ist dies aber kein Nachteil, sondern vielmehr ein großer Vorteil, da der Einsatz verschiedener Techniken die Abh{\"a}ngigkeit der Ergebnisse von der verwendeten Numerik reduziert und dadurch der grundlegende Mechanismus besser untersucht werden kann. Im speziellen werden in dieser Dissertation die Kuprate mit der Variationellen Clustern{\"a}herung ausgewertet, weil die Elektronen hier eine starke Wechselwirkung untereinander besitzen. Besonders die Frage eines m{\"o}glichen Klebstoffs f{\"u}r die Cooper-Paare wird ausf{\"u}hrlich diskutiert, auch mit einer Unterscheidung in retardierte und nicht-retardierte Betr{\"a}ge. Den Kupraten werden das Kobaltat NaCoO sowie Graphen gegen{\"u}bergestellt. Diese Materialien sind jedoch schwach korrelierte Systeme, so dass hier die Funkionelle Renormierungsgruppe als numerisches Grundger{\"u}st dient. Die Ergebnisse sind reichhaltige Phasendiagramme mit vielen verschiedenen langreichweitigen Ordnungen, wie zum Beispiel d+id-wellenartige Supraleitung. Diese bricht die Zeitumkehr-Symmetrie und besitzt eine vollst{\"a}ndige Bandl{\"u}cke, welche im Falle von NaCoO jedoch eine stark Dotierungs-abh{\"a}ngige Anisotropie aufweist. Als letztes wird das Kagome-Gitter allgemein diskutiert, ohne ein konkretes Material zu beschreiben. Hier hat eine destruktive Interferenz zwischen den Elektronen auf verschiedenen Untergittern drastische Auswirkungen auf die Instabilit{\"a}ten der Fermi-Fl{\"a}che, so dass die {\"u}bliche Spin-Dichte-Welle und die damit verbundene d+id-wellenartige Supraleitung unterdr{\"u}ckt werden. Dadurch treten ungew{\"o}hnliche Spin- und Ladungsdichte-Ordnungen sowie eine nematische Pomeranchuck Instabilit{\"a}t hervor. Zusammengefasst bietet diese Dissertation einen Einblick in unterschiedliche Materialklassen von unkonventionellen Supraleitern. Dadurch wird es m{\"o}glich, die Material-spezifischen Eigenschaften von den universellen zu trennen.}, subject = {Supraleitung}, language = {en} } @phdthesis{Budich2012, author = {Budich, Jan Carl}, title = {Fingerprints of Geometry and Topology on Low Dimensional Mesoscopic Systems}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-76847}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2012}, abstract = {In this PhD thesis, the fingerprints of geometry and topology on low dimensional mesoscopic systems are investigated. In particular, holographic non-equilibrium transport properties of the quantum spin Hall phase, a two dimensional time reversal symmetric bulk insulating phase featuring one dimensional gapless helical edge modes are studied. In these metallic helical edge states, the spin and the direction of motion of the charge carriers are locked to each other and counter-propagating states at the same energy are conjugated by time reversal symmetry. This phenomenology entails a so called topological protection against elastic single particle backscattering by time reversal symmetry. We investigate the limitations of this topological protection by studying the influence of inelastic processes as induced by the interplay of phonons and extrinsic spin orbit interaction and by taking into account multi electron processes due to electron-electron interaction, respectively. Furthermore, we propose possible spintronics applications that rely on a spin charge duality that is uniquely associated with the quantum spin Hall phase. This duality is present in the composite system of two helical edge states with opposite helicity as realized on the two opposite edges of a quantum spin Hall sample with ribbon geometry. More conceptually speaking, the quantum spin Hall phase is the first experimentally realized example of a symmetry protected topological state of matter, a non-interacting insulating band structure which preserves an anti-unitary symmetry and is topologically distinct from a trivial insulator in the same symmetry class with totally localized and hence independent atomic orbitals. In the first part of this thesis, the reader is provided with a fairly self-contained introduction into the theoretical concepts underlying the timely research field of topological states of matter. In this context, the topological invariants characterizing these novel states are viewed as global analogues of the geometric phase associated with a cyclic adiabatic evolution. Whereas the detailed discussion of the topological invariants is necessary to gain deeper insight into the nature of the quantum spin Hall effect and related physical phenomena, the non-Abelian version of the local geometric phase is employed in a proposal for holonomic quantum computing with spin qubits in quantum dots.}, subject = {Topologischer Isolator}, language = {en} } @article{Winter2012, author = {Winter, Walter}, title = {Neutrinos from Cosmic Accelerators Including Magnetic Field and Flavor Effects}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-75290}, year = {2012}, abstract = {We review the particle physics ingredients affecting the normalization, shape, and flavor composition of astrophysical neutrinos fluxes, such as different production modes, magnetic field effects on the secondaries muons, pions, and kaons, and flavor mixing, where we focus on p? interactions. We also discuss the interplay with neutrino propagation and detection, including the possibility to detect flavor and its application in particle physics, and the use of the Glashow resonance to discriminate p? from pp interactions in the source. We illustrate the implications on fluxes and flavor composition with two different models: 1 the target photon spectrum is dominated by synchrotron emission of coaccelerated electrons and 2 the target photon spectrum follows the observed photon spectrum of gamma-ray bursts. In the latter case, the multimessenger extrapolation from the gamma-ray fluence to the expected neutrino flux is highlighted.}, subject = {Magnetfeld}, language = {en} } @phdthesis{Walter2012, author = {Walter, Stefan}, title = {Exploring the Quantum Regime of Nanoelectromechanical Systems}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-75188}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2012}, abstract = {This thesis deals with nanoelectromechanical systems in the quantum regime. Nanoelectromechanical systems are systems where a mechanical degree of freedom of rather macroscopic size is coupled to an electronic degree of freedom. The mechanical degree of freedom can without any constraints be modeled as the fundamental mode of a harmonic oscillator. Due to their size and the energy scales involved in the setting, quantum mechanics plays an important role in their description. We investigate transport through such nanomechanical devices where our focus lies on the quantum regime. We use non-equilibrium methods to fully cover quantum effects in setups where the mechanical oscillator is part of a tunnel junction. In such setups, the mechanical motion influences the tunneling amplitude and thereby the transport properties through the device. The electronics in these setups can then be used to probe and characterize the mechanical oscillator through signatures in transport quantities such as the average current or the current noise. The interplay between the mechanical motion and other physical degrees of freedom can also be used to characterize these other degrees of freedom, i.e., the nanomechanical oscillator can be used as a detector. In this thesis, we will show that a nanomechanical oscillator can be used as a detector for rather exotic degrees of freedom, namely Majorana bound states which recently attracted great interest, theoretically as well as experimentally. Again, the quantum regime plays an essential role in this topic. One of the major manifestations of quantum mechanics is entanglement between two quantum systems. Entanglement of quantum systems with few (discrete) degrees of freedom is a well established and understood subject experimentally as well as theoretically. Here, we investigate quantum entanglement between two macroscopic continuous variable systems. We study different setups where it is possible to entangle two nanomechanical oscillators which are not directly coupled to each other. We conclude with reviewing the obtained results and discuss open questions and possible future developments on the quantum aspects of nanomechanical systems.}, subject = {Nanoelektromechanik}, language = {en} } @phdthesis{Luitz2012, author = {Luitz, David J.}, title = {Numerical methods and applications in many fermion systems}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-75927}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2012}, abstract = {This thesis presents results covering several topics in correlated many fermion systems. A Monte Carlo technique (CT-INT) that has been implemented, used and extended by the author is discussed in great detail in chapter 3. The following chapter discusses how CT-INT can be used to calculate the two particle Green's function and explains how exact frequency summations can be obtained. A benchmark against exact diagonalization is presented. The link to the dynamical cluster approximation is made in the end of chapter 4, where these techniques are of immense importance. In chapter 5 an extensive CT-INT study of a strongly correlated Josephson junction is shown. In particular, the signature of the first order quantum phase transition between a Kondo and a local moment regime in the Josephson current is discussed. The connection to an experimental system is made with great care by developing a parameter extraction strategy. As a final result, we show that it is possible to reproduce experimental data from a numerically exact CT-INT model-calculation. The last topic is a study of graphene edge magnetism. We introduce a general effective model for the edge states, incorporating a complicated interaction Hamiltonian and perform an exact diagonalization study for different parameter regimes. This yields a strong argument for the importance of forbidden umklapp processes and of the strongly momentum dependent interaction vertex for the formation of edge magnetism. Additional fragments concerning the use of a Legendre polynomial basis for the representation of the two particle Green's function, the analytic continuation of the self energy for the Anderson Kane Mele Model, as well as the generation of test data with a given covariance matrix are documented in the appendix. A final appendix provides some very important matrix identities that are used for the discussion of technical details of CT-INT.}, subject = {Fermionensystem}, language = {en} } @article{VarykhalovMarchenkoSanchezBarrigaetal.2012, author = {Varykhalov, A. and Marchenko, D. and S{\´a}nchez-Barriga, J. and Scholz, M. R. and Verberck, B. and Trauzettel, B. and Wehling, T. O. and Carbone, C. and Rader, O.}, title = {Intact Dirac Cones at Broken Sublattice Symmetry: Photoemission Study of Graphene on Ni and Co}, series = {Physical Review X}, volume = {2}, journal = {Physical Review X}, number = {041017}, doi = {10.1103/PhysRevX.2.041017}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-135732}, year = {2012}, abstract = {The appearance of massless Dirac fermions in graphene requires two equivalent carbon sublattices of trigonal shape. While the generation of an effective mass and a band gap at the Dirac point remains an unresolved problem for freestanding extended graphene, it is well established by breaking translational symmetry by confinement and by breaking sublattice symmetry by interaction with a substrate. One of the strongest sublattice-symmetry-breaking interactions with predicted and measured band gaps ranging from 400 meV to more than 3 eV has been attributed to the interfaces of graphene with Ni and Co, which are also promising spin-filter interfaces. Here, we apply angle-resolved photoemission to epitaxial graphene on Ni (111) and Co(0001) to show the presence of intact Dirac cones 2.8 eV below the Fermi level. Our results challenge the common belief that the breaking of sublattice symmetry by a substrate and the opening of the band gap at the Dirac energy are in a straightforward relation. A simple effective model of a biased bilayer structure composed of graphene and a sublattice-symmetry-broken layer, corroborated by density-functional-theory calculations, demonstrates the general validity of our conclusions.}, language = {en} } @article{BechtleBringmannDeschetal.2012, author = {Bechtle, Philip and Bringmann, Torsten and Desch, Klaus and Dreiner, Herbi and Hamer, Matthias and Hensel, Carsten and Kr{\"a}mer, Michael and Nguyen, Nelly and Porod, Werner and Prudent, Xavier and Sarrazin, Bj{\"o}rn and Uhlenbrock, Mathias and Wienemann, Peter}, title = {Constrained supersymmetry after two years of LHC data: a global view with Fittino}, series = {Journal of High Energy Physics}, volume = {06}, journal = {Journal of High Energy Physics}, number = {098}, doi = {10.1007/JHEP06(2012)098}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-129573}, year = {2012}, abstract = {We perform global fits to the parameters of the Constrained Minimal Super-symmetric Standard Model (CMSSM) and to a variant with non-universal Higgs masses (NUHM1). In addition to constraints from low-energy precision observables and the cosmological dark matter density, we take into account the LHC exclusions from searches in jets plus missing transverse energy signatures with about 5 fb\(^{-1}\) of integrated luminosity. We also include the most recent upper bound on the branching ratio B\(_s\)  → μμ from LHCb. Furthermore, constraints from and implications for direct and indirect dark matter searches are discussed. The best fit of the CMSSM prefers a light Higgs boson just above the experimentally excluded mass. We find that the description of the low-energy observables, (g - 2)\(_μ\) in particular, and the non-observation of SUSY at the LHC become more and more incompatible within the CMSSM. A potential SM-like Higgs boson with mass around 126 GeV can barely be accommodated. Values for B(B\(_s\)→μμ) just around the Standard Model prediction are naturally expected in the best fit region. The most-preferred region is not yet affected by limits on direct WIMP searches, but the next generation of experiments will probe this region. Finally, we discuss implications from fine-tuning for the best fit regions.}, language = {en} } @article{KilianOhlReuteretal.2012, author = {Kilian, W. and Ohl, T. and Reuter, J. and Speckner, C.}, title = {QCD in the color-flow representation}, series = {Journal of High Energy Physics}, volume = {10}, journal = {Journal of High Energy Physics}, number = {022}, doi = {10.1007/JHEP10(2012)022}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-129583}, year = {2012}, abstract = {For many practical purposes, it is convenient to formulate unbroken non-abelian gauge theories like QCD in a color-flow basis. We present a new derivation of SU(N) interactions in the color-flow basis by extending the gauge group to U(N) × U(1)′ in such a way that the two U(1) factors cancel each other. We use the quantum action principles to show the equivalence to the usual basis to all orders in perturbation theory. We extend the known Feynman rules to exotic color representations (e.g. sextets) and discuss practical applications as they occur in automatic computation programs.}, language = {en} } @article{LangenfeldMochPfoh2012, author = {Langenfeld, Ulrich and Moch, Sven-Olaf and Pfoh, Torsten}, title = {QCD threshold corrections for gluino pair production at hadron colliders}, series = {Journal of High Energy Physics}, volume = {11}, journal = {Journal of High Energy Physics}, number = {070}, doi = {10.1007/JHEP11(2012)070}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-129609}, year = {2012}, abstract = {We present the complete threshold enhanced predictions in QCD for the total cross section of gluino pair production at hadron colliders at next-to-next-to-leading order. Thanks to the computation of the required one-loop hard matching coefficients our results are accurate to the next-to-next-to-leading logarithm. In a brief phenomenological study we provide predictions for the total hadronic cross sections at the LHC and we discuss the uncertainties arising from scale variations and the parton distribution functions.}, language = {en} } @phdthesis{Ruegamer2012, author = {R{\"u}gamer, Stefan}, title = {Multi-Wavelength Observations of the high-peaked BL Lacertae objects 1ES 1011+496 and 1ES 2344+514}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-77846}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2012}, abstract = {BL Lacertae objects belong to the most luminous sources in the Universe. They represent a subclass of active galactic nuclei with a spectrum that is dominated by non-thermal emission, extending from radio wavelengths to tera electronvolt (TeV) energies. The emission is strongly variable on time scales of years down to minutes, and arises from relativistic jets pointing at small angles to the line of sight of the observer, which is the reason for naming them "blazars". Blazars are the dominant extragalactic source class in the radio, microwave and gamma-ray regime, are prime candidates for the origin of the Cosmic Rays and excellent laboratories to study black hole and jet physics as well as relativistic effects. Despite more than 20 years of observational efforts, the physical mechanisms driving their emission are not yet fully understood. So far, studies of their broad-band continuum emission were mostly concentrated on bright, flaring states. However, for a better understanding of the central engine powering the jets, the bias from flux-limited observations of the past must be overcome and their long-term average continuum spectral energy distributions (SEDs) must be determined. This work presents the first simultaneous multi-wavelength campaigns from the radio to the TeV regime of two high-frequency peaked BL Lacertae objects known to emit at TeV energies. The first source, 1ES 1011+496, was observed between February and May 2008, the second one, 1ES 2344+514, between September 2008 and February 2009. The extensive observational campaigns were organised independently from an external trigger for the presence of a flaring state. Since the duty cycle of major flux outbursts is known to be rather low, the campaigns were expected to yield SEDs representative of the long-term average emission. Central for this thesis is the analysis of data obtained with the MAGIC Cherenkov telescope, measuring energy spectra and light curves from ~0.1 to ~10 TeV. For the remaining instruments, observation time was proposed and additional data was organised by collaboration with the instrument teams by the author of this work. Such data was obtained mostly in a fully reduced state. Individual light curves are investigated as well as combined in a search for inter-band correlations. The data of both sources reveal a notable lack of a correlation between the emission at radio and optical wavelengths, indicating that the radio and short-wavelength emission arise in different regions of the jet. Quasi-simultaneous SEDs of two different flux states are observationally determined and described by a one-zone as well as a self-consistent two-zone synchrotron self-Compton model. First approaches to model the SEDs by means of a Chi2 minimisation technique are briefly discussed. The SEDs and the resulting model parameters, characterising the physical conditions in the emission regions, are compared to archival data. Though the models can describe the data well, for 1ES 1011+496 the model parameters indicate that in addition to the synchrotron and inverse-Compton emission of relativistic electrons, emission due to accelerated protons seems to be required. The SEDs of 1ES 2344+514 reveal one of the lowest activity states ever detected from the source. Despite that, the model parameters are not indicative of a distinct quiescent state, which may be caused by the degeneracy of the different parameters in one-zone models. Moreover, indications accumulate that the radiation can not be attributed to a single emission region. The results disfavour some of the current blazar classification schemes and the so-called "blazar sequence", emphasising the need for a more realistic explanation of the systematics of the blazar SEDs in terms of fundamental parameters.}, subject = {Blazar}, language = {en} } @phdthesis{Heiligenthal2012, author = {Heiligenthal, Sven}, title = {Strong and Weak Chaos in Networks of Semiconductor Lasers with Time-Delayed Couplings}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-77958}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2012}, abstract = {This thesis deals with the chaotic dynamics of nonlinear networks consisting of semiconductor lasers which have time-delayed self-feedbacks or mutual couplings. These semiconductor lasers are simulated numerically by the Lang-Kobayashi equations. The central issue is how the chaoticity of the lasers, measured by the maximal Lyapunov exponent, changes when the delay time is changed. It is analysed how this change of chaoticity with increasing delay time depends on the reflectivity of the mirror for the self-feedback or the strength of the mutal coupling, respectively. The consequences of the different types of chaos for the effect of chaos synchronization of mutually coupled semiconductor lasers are deduced and discussed. At the beginning of this thesis, the master stability formalism for the stability analysis of nonlinear networks with delay is explained. After the description of the Lang-Kobayashi equations and their linearizations as a model for the numerical simulation of semiconductor lasers with time-delayed couplings, the artificial sub-Lyapunov exponent \$\lambda_{0}\$ is introduced. It is explained how the sign of the sub-Lyapunov exponent can be determined by experiments. The notions of "strong chaos" and "weak chaos" are introduced and distinguished by their different scaling properties of the maximal Lyapunov exponent with the delay time. The sign of the sub-Lyapunov exponent \$\lambda_{0}\$ is shown to determine the occurence of strong or weak chaos. The transition sequence "weak to strong chaos and back to weak chaos" upon monotonically increasing the coupling strength \$\sigma\$ of a single laser's self-feedback is shown for numerical calculations of the Lang-Kobayashi equations. At the transition between strong and weak chaos, the sub-Lyapunov exponent vanishes, \$\lambda_{0}=0\$, resulting in a special scaling behaviour of the maximal Lyapunov exponent with the delay time. Transitions between strong and weak chaos by changing \$\sigma\$ can also be found for the R{\"o}ssler and Lorenz dynamics. The connection between the sub-Lyapunov exponent and the time-dependent eigenvalues of the Jacobian for the internal laser dynamics is analysed. Counterintuitively, the difference between strong and weak chaos is not directly visible from the trajectory although the difference of the trajectories induces the transitions between the two types of chaos. In addition, it is shown that a linear measure like the auto-correlation function cannot unambiguously reveal the difference between strong and weak chaos either. Although the auto-correlations after one delay time are significantly higher for weak chaos than for strong chaos, it is not possible to detect a qualitative difference. If two time-scale separated self-feedbacks are present, the shorter feedback has to be taken into account for the definition of a new sub-Lyapunov exponent \$\lambda_{0,s}\$, which in this case determines the occurence of strong or weak chaos. If the two self-feedbacks have comparable delay times, the sub-Lyapunov exponent \$\lambda_{0}\$ remains the criterion for strong or weak chaos. It is shown that the sub-Lyapunov exponent scales with the square root of the effective pump current \$\sqrt{p-1}\$, both in its magnitude and in the position of the critical coupling strengths. For networks with several distinct sub-Lyapunov exponents, it is shown that the maximal sub-Lyapunov exponent of the network determines whether the network's maximal Lyapunov exponent scales strongly or weakly with increasing delay time. As a consequence, complete synchronization of a network is excluded for arbitrary networks which contain at least one strongly chaotic laser. Furthermore, it is demonstrated that the sub-Lyapunov exponent of a driven laser depends on the number of the incoherently superimposed inputs from unsynchronized input lasers. For networks of delay-coupled lasers operating in weak chaos, the condition \$|\gamma_{2}|<\mathrm{e}^{-\lambda_{\mathrm{m}}\,\tau}\$ for stable chaos synchronization is deduced using the master stability formalism. Hence, synchronization of any network depends only on the properties of a single laser with self-feedback and the eigenvalue gap of the coupling matrix. The characteristics of the master stability function for the Lang-Kobayashi dynamics is described, and consequently, the master stability function is refined to allow for precise practical prediction of synchronization. The prediction of synchronization with the master stability function is demonstrated for bidirectional and unidirectional networks. Furthermore, the master stability function is extended for two distinct delay times. Finally, symmetries and resonances for certain values of the ratio of the delay times are shown for the master stability function of the Lang-Kobyashi equations.}, subject = {Halbleiterlaser}, language = {en} } @phdthesis{Platt2012, author = {Platt, Christian}, title = {A Common Thread in Unconventional Superconductivity: The Functional Renormalization Group in Multi-Band Systems}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-78824}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2012}, abstract = {Die supraleitenden Eigenschaften von komplexen Materialsystemen, wie den erst k{\"u}rzlich entdeckten Eisen-Pniktiden oder den Strontium-Ruthenaten, sind oftmals durch das Zusammenspiel vieler elektronischer Orbitale bestimmt. Um die Supraleitung in derartigen Systemen besser zu verstehen, entwickeln wir in dieser Arbeit eine Multi-Orbital-Implementierung der funktionalen Renormierungsgruppe und untersuchen die Elektronenpaarung in verschiedenen charakteristischen Materialverbindungen. In den Eisen-Pniktiden finden wir hierbei mehrere Spinfluktuationskan{\"a}le, die eine Elektronenpaarung hervorrufen, sofern die Paarwellenfunktion einen Vorzeichenwechsel zwischen den verschiedenen genesteten Bereichen der Fermifl{\"a}che aufweist. Abh{\"a}ngig von den spezifischen Materialeigenschaften, wie der Dotierung oder der Position des Pniktogenatoms, f{\"u}hren diese Spinfluktuationen dann zu \$s_{\pm}\$-wellenartiger Paarung mit durchg{\"a}ngiger Energiel{\"u}cke oder mit Knoten auf der Fermifl{\"a}che. In manchen F{\"a}llen wird zudem auch \$d\$-wellenartige Paarung induziert, die in der N{\"a}he des {\"U}bergangs zur \$s_{\pm}\$-Symmetrie einen gemischten \$(s+id)\$-Zustand mit gebrochener Zeitinversionssymmetrie aufweist. Diese neuartige Phase zeigt faszinierende Eigenschaften, wie zum Beispiel das spontane Entstehen von Suprastr{\"o}men am Probenrand und um nichtmagnetische St{\"o}rstellen. Auf Grund der durchg{\"a}ngigen Energiel{\"u}cke ist dieser \$(s+id)\$-Zustand energetisch beg{\"u}nstigt. Im Folgenden untersuchen wir zudem auch die elektronischen Instabilit{\"a}ten eines weiteren außergew{\"o}hnlichen Materials -- dotiertes Graphen. Diese rein zweidimensionale Kohlenstoffverbindung ist schon seit mehreren Jahren im Fokus der Festk{\"o}rperforschung und wurde mittlerweile auch durch neuartige experimentelle Verfahren dotiert, ohne die zugrundeliegende hexagonale Gittersturktur merklich zu st{\"o}ren. Eine theoretische Beschreibung dieses Systems erfordert die Ber{\"u}cksichtigung zweier nicht-equivalenter Gitterpl{\"a}tze, was wiederum effektiv als Zwei-Orbital-System aufgefasst werden kann. Durch die besondere Symmetrie der hexagonalen Gitterstruktur sind beide \$d\$-wellenartigen Paarungskan{\"a}le entartet und ahnlich der \$(s+id)\$-Paarung in den Pniktiden finden wir hier eine chirale \$(d+id)\$-Paarung in einem weiten Dotierungsbereich um van-Hove F{\"u}llung. Des Weiteren identifizieren wir Spin-Triplet-Paarung und eine exotische Form der Spindichtewelle, welche beide durch leichte Ver{\"a}nderung der langreichweitigen H{\"u}pfamplituden und Wechselwirkungensparameter realisiert werden k{\"o}nnen. Als drittes Beispiel betrachten wir die Supraleitung in dem Strontium-Ruthenat Sr\$_2\$RuO\$_4\$. Die Besonderheit dieser Materialverbindung liegt in der m{\"o}glichen Realisierung einer chiralen Spin-Triplet Paarung, die wiederum faszinierende Eigenschaften wie die Existenz von halbganzzahligen Flussvortizes mit nicht-Abelscher Vertauschungsstatistik aufweisen w{\"u}rde. Mittels eines mikroskopischen Drei-Orbital-Modells und der Ber{\"u}cksichtigung von Spin-Bahn-Kopplung finden wir hierbei, dass moderate ferromagnetische Spinfluktuationen immer noch ausreichen, um diesen speziellen Paarungszustand anzutreiben. Die berechnete Energiel{\"u}cke zeigt im Weiteren sehr starke Anisotropien auf dem \$d_{xy}\$-Orbital-dominierten Bereich der Fermifl{\"a}che und verschwindet nahezu vollst{\"a}ndig auf den anderen beiden Fermifl{\"a}chen.}, subject = {Supraleitung}, language = {en} } @article{GedalinDroege2013, author = {Gedalin, Michael and Dr{\"o}ge, Wolfgang}, title = {Ion dynamics in quasi-perpendicular collisionless interplanetary shocks: a case study}, series = {Frontiers in Physics}, volume = {1}, journal = {Frontiers in Physics}, issn = {2296-424X}, doi = {10.3389/fphy.2013.00029}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-189407}, pages = {29}, year = {2013}, abstract = {Interplanetary shocks are believed to play an important role in the acceleration of charged particles in the heliosphere. While the acceleration to high energies proceeds via the diffusive mechanism at the scales exceeding by far the shock width, the initial stage (injection) should occur at the shock itself. Numerical tracing of ions is done in a model quasi-perpendicular shock front with a typical interplanetary shock parameters (Mach number, upstream ion temperature). The analysis of the distribution of the transmitted solar wind is used to adjust the cross-shock potential which is not directly measured. It is found that, for typical upstream ion temperatures, acceleration of the ions from the tail of the solar wind distribution is unlikely. Pickup ions with a shell distribution are found to be effectively energized and may be injected into further diffusive acceleration regime. Pre-accelerated ions are efficiently upscaled in energies. A part of these ions is returned to the upstream region where they can further be diffusively accelerated.}, language = {en} } @article{VainioValtonenHeberetal.2013, author = {Vainio, Rami and Valtonen, Eino and Heber, Bernd and Malandraki, Olga E. and Papaioannou, Athanasios and Klein, Karl-Ludwig and Afanasiev, Alexander and Agueda, Neus and Aurass, Henry and Battarbee, Markus and Braune, Stephan and Dr{\"o}ge, Wolfgang and Ganse, Urs and Hamadache, Clarisse and Heynderickx, Daniel and Huttunen-Heikinmaa, Kalle and Kiener, J{\"u}rgen and Kilian, Patrick and Kopp, Andreas and Kouloumvakos, Athanasios and Maisala, Sami and Mishev, Alexander and Miteva, Rosita and Nindos, Alexander and Oittinen, Tero and Raukunen, Osku and Riihonen, Esa and Rodriguez-Gasen, Rosa and Saloniemi, Oskari and Sanahuja, Blai and Scherer, Renate and Spanier, Felix and Tatischeff, Vincent and Tziotziou, Kostas and Usoskin, Ilya G. and Vilmer, Nicole}, title = {The first SEPServer event catalogue similar to ~68-MeV solar proton events observed at 1 AU in 1996-2010}, series = {Journal of Space Weather and Space Climate}, volume = {3}, journal = {Journal of Space Weather and Space Climate}, number = {A12}, issn = {2115-7251}, doi = {10.1051/swsc/2013030}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-122847}, year = {2013}, abstract = {SEPServer is a three-year collaborative project funded by the seventh framework programme (FP7-SPACE) of the European Union. The objective of the project is to provide access to state-of-the-art observations and analysis tools for the scientific community on solar energetic particle (SEP) events and related electromagnetic (EM) emissions. The project will eventually lead to better understanding of the particle acceleration and transport processes at the Sun and in the inner heliosphere. These processes lead to SEP events that form one of the key elements of space weather. In this paper we present the first results from the systematic analysis work performed on the following datasets: SOHO/ERNE, SOHO/EPHIN, ACE/EPAM, Wind/WAVES and GOES X-rays. A catalogue of SEP events at 1 AU, with complete coverage over solar cycle 23, based on high-energy (similar to 68-MeV) protons from SOHO/ERNE and electron recordings of the events by SOHO/EPHIN and ACE/EPAM are presented. A total of 115 energetic particle events have been identified and analysed using velocity dispersion analysis (VDA) for protons and time-shifting analysis (TSA) for electrons and protons in order to infer the SEP release times at the Sun. EM observations during the times of the SEP event onset have been gathered and compared to the release time estimates of particles. Data from those events that occurred during the European day-time, i.e., those that also have observations from ground-based observatories included in SEPServer, are listed and a preliminary analysis of their associations is presented. We find that VDA results for protons can be a useful tool for the analysis of proton release times, but if the derived proton path length is out of a range of 1 AU < s less than or similar to 3 AU, the result of the analysis may be compromised, as indicated by the anti-correlation of the derived path length and release time delay from the associated X-ray flare. The average path length derived from VDA is about 1.9 times the nominal length of the spiral magnetic field line. This implies that the path length of first-arriving MeV to deka-MeV protons is affected by interplanetary scattering. TSA of near-relativistic electrons results in a release time that shows significant scatter with respect to the EM emissions but with a trend of being delayed more with increasing distance between the flare and the nominal footpoint of the Earth-connected field line.}, language = {en} } @article{EdgecockCarettaDavenneetal.2013, author = {Edgecock, T. R. and Caretta, O. and Davenne, T. and Densam, C. and Fitton, M. and Kelliher, D. and Loveridge, P. and Machida, S. and Prior, C. and Rogers, C. and Rooney, M. and Thomason, J. and Wilcox, D. and Wildner, E. and Efthymiopoulos, I. and Garoby, R. and Gilardoni, S. and Hansen, C. and Benedetto, E. and Jensen, E. and Kosmicki, A. and Martini, M. and Osborne, J. and Prior, G. and Stora, T. and Melo Mendonca, T. and Vlachoudis, V. and Waaijer, C. and Cupial, P. and Chanc{\´e}, A. and Longhin, A. and Payet, J. and Zito, M. and Baussan, E. and Bobeth, C. and Bouquerel, E. and Dracos, M. and Gaudiot, G. and Lepers, B. and Osswald, F. and Poussot, P. and Vassilopoulos, N. and Wurtz, J. and Zeter, V. and Bielski, J. and Kozien, M. and Lacny, L. and Skoczen, B. and Szybinski, B. and Ustrycka, A. and Wroblewski, A. and Marie-Jeanne, M. and Balint, P. and Fourel, C. and Giraud, J. and Jacob, J. and Lamy, T. and Latrasse, L. and Sortais, P. and Thuillier, T. and Mitrofanov, S. and Loiselet, M. and Keutgen, Th. and Delbar, Th. and Debray, F. and Trophine, C. and Veys, S. and Daversin, C. and Zorin, V. and Izotov, I. and Skalyga, V. and Burt, G. and Dexter, A. C. and Kravchuk, V. L. and Marchi, T. and Cinausero, M. and Gramegna, F. and De Angelis, G. and Prete, G. and Collazuol, G. and Laveder, M. and Mazzocco, M. and Mezzetto, M. and Signorini, C. and Vardaci, E. and Di Nitto, A. and Brondi, A. and La Rana, G. and Migliozzi, P. and Moro, R. and Palladino, V. and Gelli, N. and Berkovits, D. and Hass, M. and Hirsh, T. Y. and Schuhmann, M. and Stahl, A. and Wehner, J. and Bross, A. and Kopp, J. and Neuffer, D. and Wands, R. and Bayes, R. and Laing, A. and Soler, P. and Agarwalla, S. K. and Cervera Villanueva, A. and Donini, A. and Ghosh, T. and G{\´o}mez Cadenas, J. J. and Hern{\´a}ndez, P. and Mart{\´i}n-Albo, J. and Mena, O. and Burguet-Castell, J. and Agostino, L. and Buizza-Avanzini, M. and Marafini, M. and Patzak, T. and Tonazzo, A. and Duchesneau, D. and Mosca, L. and Bogomilov, M. and Karadzhov, Y. and Matev, R. and Tsenov, R. and Akhmedov, E. and Blennow, M. and Lindner, M. and Schwetz, T. and Fern{\´a}ndez Martinez, E. and Maltoni, M. and Men{\´e}ndez, J. and Giunti, C. and Gonz{\´a}lez Garc{\´i}a, M. C. and Salvado, J. and Coloma, P. and Huber, P. and Li, T. and L{\´o}pez Pav{\´o}n, J. and Orme, C. and Pascoli, S. and Meloni, D. and Tang, J. and Winter, W. and Ohlsson, T. and Zhang, H. and Scotto-Lavina, L. and Terranova, F. and Bonesini, M. and Tortora, L. and Alekou, A. and Aslaninejad, M. and Bontoiu, C. and Kurup, A. and Jenner, L. J. and Long, K. and Pasternak, J. and Pozimski, J. and Back, J. J. and Harrison, P. and Beard, K. and Bogacz, A. and Berg, J. S. and Stratakis, D. and Witte, H. and Snopok, P. and Bliss, N. and Cordwell, M. and Moss, A. and Pattalwar, S. and Apollonio, M.}, title = {High intensity neutrino oscillation facilities in Europe}, series = {Physical Review Special Topics-Accelerators and Beams}, volume = {16}, journal = {Physical Review Special Topics-Accelerators and Beams}, number = {2}, doi = {10.1103/PhysRevSTAB.16.021002}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-126611}, pages = {21002}, year = {2013}, abstract = {The EUROnu project has studied three possible options for future, high intensity neutrino oscillation facilities in Europe. The first is a Super Beam, in which the neutrinos come from the decay of pions created by bombarding targets with a 4 MW proton beam from the CERN High Power Superconducting Proton Linac. The far detector for this facility is the 500 kt MEMPHYS water Cherenkov, located in the Frejus tunnel. The second facility is the Neutrino Factory, in which the neutrinos come from the decay of mu(+) and mu(-) beams in a storage ring. The far detector in this case is a 100 kt magnetized iron neutrino detector at a baseline of 2000 km. The third option is a Beta Beam, in which the neutrinos come from the decay of beta emitting isotopes, in particular He-6 and Ne-18, also stored in a ring. The far detector is also the MEMPHYS detector in the Frejus tunnel. EUROnu has undertaken conceptual designs of these facilities and studied the performance of the detectors. Based on this, it has determined the physics reach of each facility, in particular for the measurement of CP violation in the lepton sector, and estimated the cost of construction. These have demonstrated that the best facility to build is the Neutrino Factory. However, if a powerful proton driver is constructed for another purpose or if the MEMPHYS detector is built for astroparticle physics, the Super Beam also becomes very attractive.}, language = {en} } @article{CamagoMolinaO'LearyPorodetal.2013, author = {Camago-Molina, J.E. and O'Leary, B. and Porod, W. and Staub, F.}, title = {Vevacious: a tool for finding the global minima of one-loop effective potentials with many scalars}, series = {European Physical Journal C}, volume = {73}, journal = {European Physical Journal C}, number = {2588}, doi = {10.1140/epjc/s10052-013-2588-2}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-132110}, year = {2013}, abstract = {Several extensions of the Standard Model of particle physics contain additional scalars implying a more complex scalar potential compared to that of the Standard Model. In general these potentials allow for charge- and/or color-breaking minima besides the desired one with correctly broken SU(2) L ×U(1) Y . Even if one assumes that a metastable local minimum is realized, one has to ensure that its lifetime exceeds that of our universe. We introduce a new program called Vevacious which takes a generic expression for a one-loop effective potential energy function and finds all the tree-level extrema, which are then used as the starting points for gradient-based minimization of the one-loop effective potential. The tunneling time from a given input vacuum to the deepest minimum, if different from the input vacuum, can be calculated. The parameter points are given as files in the SLHA format (though is not restricted to supersymmetric models), and new model files can be easily generated automatically by the Mathematica package SARAH. This code uses HOM4PS2 to find all the minima of the tree-level potential, PyMinuit to follow gradients to the minima of the one-loop potential, and CosmoTransitions to calculate tunneling times.}, language = {en} } @article{Winter2013, author = {Winter, Walter}, title = {Long-baseline sensitivity studies and comparison (discussion session)}, series = {Journal of Physics: Conference Series}, volume = {408}, journal = {Journal of Physics: Conference Series}, number = {012020}, doi = {10.1088/1742-6596/408/1/012020}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-129440}, year = {2013}, abstract = {In this discussion session, the sensitivity and optimization of future long-baseline experiments is addressed, with a special emphasis on feasible projects and the description in terms of the error on the parameters. In addition, a statement on the precision interesting for \(ν_e → ν_τ\) and \(ν_μ → ν_τ\)oscillation measurements is obtained. A special topic is the impact of the recent T2K hint for non-zero \(θ_{13}\).}, language = {en} } @article{AssaadHerbut2013, author = {Assaad, Fakher F. and Herbut, Igor F.}, title = {Pinning the Order: The Nature of Quantum Criticality in the Hubbard Model on Honeycomb Lattice}, series = {Physical Review X}, volume = {3}, journal = {Physical Review X}, number = {031010}, doi = {10.1103/PhysRevX.3.031010}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-129829}, year = {2013}, abstract = {In numerical simulations, spontaneously broken symmetry is often detected by computing two-point correlation functions of the appropriate local order parameter. This approach, however, computes the square of the local order parameter, and so when it is small, very large system sizes at high precisions are required to obtain reliable results. Alternatively, one can pin the order by introducing a local symmetrybreaking field and then measure the induced local order parameter infinitely far from the pinning center. The method is tested here at length for the Hubbard model on honeycomb lattice, within the realm of the projective auxiliary-field quantum Monte Carlo algorithm. With our enhanced resolution, we find a direct and continuous quantum phase transition between the semimetallic and the insulating antiferromagnetic states with increase of the interaction. The single-particle gap, measured in units of Hubbard U, tracks the staggered magnetization. An excellent data collapse is obtained by finite-size scaling, with the values of the critical exponents in accord with the Gross-Neveu universality class of the transition.}, language = {en} } @article{AssaadBercxHohenadler2013, author = {Assaad, F. F. and Bercx, M. and Hohenadler, M.}, title = {Topological Invariant and Quantum Spin Models from Magnetic pi Fluxes in Correlated Topological Insulators}, series = {Physical Review X}, volume = {3}, journal = {Physical Review X}, number = {1}, doi = {10.1103/PhysRevX.3.011015}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-129849}, year = {2013}, abstract = {The adiabatic insertion of a \(\pi\) flux into a quantum spin Hall insulator gives rise to localized spin and charge fluxon states. We demonstrate that \(\pi\) fluxes can be used in exact quantum Monte Carlo simulations to identify a correlated \(Z_2\) topological insulator using the example of the Kane-Mele-Hubbard model. In the presence of repulsive interactions, a \(\pi\) flux gives rise to a Kramers doublet of spin-fluxon states with a Curie-law signature in the magnetic susceptibility. Electronic correlations also provide a bosonic mode of magnetic excitons with tunable energy that act as exchange particles and mediate a dynamical interaction of adjustable range and strength between spin fluxons. \(\pi\) fluxes can therefore be used to build models of interacting spins. This idea is applied to a three-spin ring and to one-dimensional spin chains. Because of the freedom to create almost arbitrary spin lattices, correlated topological insulators with \(\pi\) fluxes represent a novel kind of quantum simulator, potentially useful for numerical simulations and experiments.}, language = {en} } @article{GeisslerBudichTrauzettel2013, author = {Geissler, F. and Budich, J. C. and Trauzettel, B.}, title = {Group theoretical and topological analysis of the quantum spin Hall effect in silicene}, series = {New Journal of Physics}, volume = {15}, journal = {New Journal of Physics}, number = {085030}, doi = {10.1088/1367-2630/15/8/085030}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-129732}, year = {2013}, abstract = {Silicene consists of a monolayer of silicon atoms in a buckled honeycomb structure. It was recently discovered that the symmetry of such a system allows for interesting Rashba spin-orbit effects. A perpendicular electric field is able to couple to the sublattice pseudospin, making it possible to electrically tune and close the band gap. Therefore, external electric fields may generate a topological phase transition from a topological insulator to a normal insulator (or semimetal) and vice versa. The contribution of the present paper to the study of silicene is twofold. Firstly, we perform a group theoretical analysis to systematically construct the Hamiltonian in the vicinity of the K points of the Brillouin zone and find an additional, electric field induced spin-orbit term, that is allowed by symmetry. Subsequently, we identify a tight-binding model that corresponds to the group theoretically derived Hamiltonian near the K points. Secondly, we start from this tight-binding model to analyze the topological phase diagram of silicene by an explicit calculation of the Z2 topological invariant of the band structure. To this end, we calculate the Z2 topological invariant of the honeycomb lattice in a manifestly gauge invariant way which allows us to include Sz symmetry breaking terms—like Rashba spin-orbit interaction—into the topological analysis. Interestingly, we find that the interplay of a Rashba and an intrinsic spin-orbit term can generate a non-trivial quantum spin Hall phase in silicene. This is in sharp contrast to the more extensively studied honeycomb system graphene where Rashba spin-orbit interaction is known to compete with the quantum spin Hall effect in a detrimental way.}, language = {en} } @article{BudichTrauzettel2013, author = {Budich, Jan Carl and Trauzettel, Bj{\"o}rn}, title = {Z(2) Green's function topology of Majorana wires}, series = {New Journal of Physics}, volume = {15}, journal = {New Journal of Physics}, number = {065006}, doi = {10.1088/1367-2630/15/6/065006}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-129751}, year = {2013}, abstract = {We represent the Z2 topological invariant characterizing a one-dimensional topological superconductor using a Wess-Zumino-Witten dimensional extension. The invariant is formulated in terms of the single-particle Green's function which allows us to classify interacting systems. Employing a recently proposed generalized Berry curvature method, the topological invariant is represented independent of the extra dimension requiring only the single-particle Green's function at zero frequency of the interacting system. Furthermore, a modified twisted boundary conditions approach is used to rigorously define the topological invariant for disordered interacting systems.}, language = {en} } @phdthesis{Huemmer2013, author = {H{\"u}mmer, Svenja}, title = {Neutrinos aus photohadronischen Wechselwirkungen in kosmischen Beschleunigern}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-77519}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2013}, abstract = {In dieser Arbeit untersuchen wir die Produktion von Neutrinos in astrophysikalischen Quellen. Bei der Beschreibung der Wechselwirkung betrachten wir resonante, direkte und Multipion-Produktion. Zus{\"a}tzlich ber{\"u}cksichtigen wir die Produktion von Neutronen und positiv geladenen Kaonen. Wir beachten explizit die Energieverluste der Sekund{\"a}rteilchen - Pionen, Myonen und Kaonen - auf Grund von Synchrotronstrahlung derselben und adiabatischer Expansion. In Bezug auf den Neutrinofluss ber{\"u}cksichtigen wir Flavor-Mischungen der Neutrinos auf dem Weg zum Beobachter. Zun{\"a}chst f{\"u}hren wir eine Analyse basierend auf einem generischen Quellmodell durch, in der wir den Einfluss von Magnetfeld und Gr{\"o}ße der Quelle auf die Neutrinospektren und das Verh{\"a}ltnis der verschiedenen Neutrino-Flavor untersuchen. Es stellt sich heraus, dass man im Rahmen dieses generischen Modells verschiedene Regionen im Parameterraum anhand des Flavor-Verh{\"a}ltnisses, das f{\"u}r hohe Magnetfelder von dem zumeist angenommenen Verh{\"a}ltnis (nu_e:nu_mu:nu_tau)=(1:2:0) abweicht, klassifizieren kann. In einer zweiten Analyse bestimmen wir die erwarteten Neutrinospektren von Gammablitzen im Rahmen des Feuerball-Modells aus beobachteten Photonspektren. Es zeigt sich, dass auf Grund grober Absch{\"a}tzungen in der Literatur, der Neutrinofluss zumeist um etwa eine Gr{\"o}ßenordnung {\"u}bersch{\"a}tzt wird. Deshalb berechnen wir den erwarteten Neutrinofluss der Gammablitze neu, die w{\"a}hrend der 40-Leinen-Konfiguration des IceCube-Detektors gemessen wurden, und folgern, dass entgegen der Behauptung der IceCube-Kollaboration, das Feuerball-Modell noch nicht ausgeschlossen ist. Des Weiteren quantifizieren wir systematische und astrophysikalische Unsicherheiten in dem vorhergesagten Neutrinofluss.}, subject = {Neutrino}, language = {de} } @phdthesis{Bach2013, author = {Bach, Fabian}, title = {Charged Current Top Quark Couplings at the LHC}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-82358}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2013}, abstract = {The top quark plays an important role in current particle physics, from a theoretical point of view because of its uniquely large mass, but also experimentally because of the large number of top events recorded by the LHC experiments ATLAS and CMS, which makes it possible to directly measure the properties of this particle, for example its couplings to the other particles of the standard model (SM), with previously unknown precision. In this thesis, an effective field theory approach is employed to introduce a minimal and consistent parametrization of all anomalous top couplings to the SM gauge bosons and fermions which are compatible with the SM symmetries. In addition, several aspects and consequences of the underlying effective operator relations for these couplings are discussed. The resulting set of couplings has been implemented in the parton level Monte Carlo event generator WHIZARD in order to provide a tool for the quantitative assessment of the phenomenological implications at present and future colliders such as the LHC or a planned international linear collider. The phenomenological part of this thesis is focused on the charged current couplings of the top quark, namely anomalous contributions to the trilinear tbW coupling as well as quartic four-fermion contact interactions of the form tbff, both affecting single top production as well as top decays at the LHC. The study includes various aspects of inclusive cross section measurements as well as differential distributions of single tops produced in the t channel, bq → tq', and in the s channel, ud → tb. We discuss the parton level modelling of these processes as well as detector effects, and finally present the prospected LHC reach for setting limits on these couplings with 10 resp. 100 fb-1 of data recorded at √s = 14 TeV.}, subject = {LHC}, language = {en} } @phdthesis{Zeeb2013, author = {Zeeb, Steffen}, title = {Chaos Synchronization in Time-Delayed Coupled Networks}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-78966}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2013}, abstract = {Die vorliegende Arbeit befasst sich mit der Untersuchung verschiedener Aspekte der Chaos Synchronisation von Netzwerken mit zeitverz{\"o}gerten Kopplungen. Ein Netzwerk aus identischen chaotischen Einheiten kann vollst{\"a}ndig und isochron synchronisieren, auch wenn der Signalaustausch einer starken Zeitverz{\"o}gerung unterliegt. Im ersten Teil der Arbeit werden Systeme mit mehreren Zeitverz{\"o}gerungen betrachtet. Dabei erstrecken sich die verschiedenen Zeitverz{\"o}gerungen jeweils {\"u}ber einen weiten Bereich an Gr{\"o}ßenordnungen. Es wird gezeigt, dass diese Zeitverz{\"o}gerungen im Lyapunov Spektrum des Systems auftreten; verschiedene Teile des Spektrums skalieren jeweils mit einer der Zeitverz{\"o}gerungen. Anhand des Skalierungsverhaltens des maximalen Lyapunov Exponenten k{\"o}nnen verschiedene Arten von Chaos definiert werden. Diese bestimmen die Synchronisationseigenschaften eines Netzwerkes und werden insbesondere wichtig bei hierarchischen Netzwerken, d.h. bei Netzwerken bestehend aus Unternetzwerken, bei welchen Signale innerhalb des Unternetzwerkes auf einer anderen Zeitskala ausgetauscht werden als zwischen verschiedenen Unternetzwerken. F{\"u}r ein solches System kann sowohl vollst{\"a}ndige als auch Unternetzwerksynchronisation auftreten. Skaliert der maximale Lyapunov Exponent mit der k{\"u}rzeren Zeitverz{\"o}gerung des Unternetzwerkes dann k{\"o}nnen nur die Elemente des Unternetzwerkes synchronisieren. Skaliert der maximale Lyapunov Exponent allerdings mit der l{\"a}ngeren Zeitverz{\"o}gerung kann das komplette Netzwerk vollst{\"a}ndig synchronisieren. Dies wird analytisch f{\"u}r die Bernoulli Abbildung und numerisch f{\"u}r die Zelt Abbildung gezeigt. Der zweite Teil befasst sich mit der Attraktordimension und ihrer {\"A}nderung am {\"U}bergang zur vollst{\"a}ndiger Chaos Synchronisation. Aus dem Lyapunov Spektrum des Systems wird die Kaplan-Yorke Dimension berechnet und es wird gezeigt, dass diese am Synchronisations{\"u}bergang aus physikalischen Gr{\"u}nden einen Sprung haben muss. Aus der Zeitreihe der Dynamik des Systems wird die Korrelationsdimension bestimmt und anschließend mit der Kaplan-Yorke Dimension verglichen. F{\"u}r Bernoulli Systeme finden wir in der Tat eine Diskontinuit{\"a}t in der Korrelationsdimension. Die St{\"a}rke des Sprungs der Kaplan-Yorke Dimension wird f{\"u}r ein Netzwerk aus Bernoulli Einheiten als Funktion der Netzwerkgr{\"o}ße berechnet. Desweiteren wird das Skalierungsverhalten der Kaplan-Yorke Dimension sowie der Kolmogoroventropie in Abh{\"a}ngigkeit der Systemgr{\"o}ße und der Zeitverz{\"o}gerung untersucht. Zu guter Letzt wird eine Verstimmung der Einheiten, d.h., ein "parameter mismatch", eingef{\"u}hrt und analysiert wie diese das Verhalten der Attraktordimension {\"a}ndert. Im dritten und letzten Teil wird die lineare Antwort eines synchronisierten chaotischen Systems auf eine kleine externe St{\"o}rung untersucht. Diese St{\"o}rung bewirkt eine Abweichung der Einheiten vom perfekt synchronisierten Zustand. Die Verteilung der Abst{\"a}nde zwischen zwei Einheiten dient als Maß f{\"u}r die lineare Antwort des Systems. Diese Verteilung sowie ihre Momente werden numerisch und f{\"u}r Spezialf{\"a}lle auch analytisch berechnet. Wir finden, dass im synchronisierten Zustand, in Abh{\"a}ngigkeit der Parameter des Systems, Verteilungen auftreten k{\"o}nnen die einem Potenzgesetz gehorchen und dessen Momente divergieren. Als weiteres Maß f{\"u}r die lineare Antwort wird die Bit Error Rate einer {\"u}bermittelten bin{\"a}ren Nachricht verwendet. The Bit Error Rate ist durch ein Integral {\"u}ber die Verteilung der Abst{\"a}nde gegeben. In dieser Arbeit wird sie vorwiegend numerisch untersucht und wir finden ein komplexes, nicht monotones Verhalten als Funktion der Kopplungsst{\"a}rke. F{\"u}r Spezialf{\"a}lle weist die Bit Error Rate eine "devil's staircase" auf, welche mit einer fraktalen Struktur in der Verteilung der Abst{\"a}nde verkn{\"u}pft ist. Die lineare Antwort des Systems auf eine harmonische St{\"o}rung wird ebenfalls untersucht. Es treten Resonanzen auf, welche in Abh{\"a}ngigkeit von der Zeitverz{\"o}gerung unterdr{\"u}ckt oder verst{\"a}rkt werden. Eine bi-direktional gekoppelte Kette aus drei Einheiten kann eine St{\"o}rung vollst{\"a}ndig heraus filtern, so dass die Bit Error Rate und auch das zweite Moment verschwinden.}, subject = {Chaostheorie}, language = {en} } @phdthesis{Parragh2013, author = {Parragh, Nicolaus}, title = {Strongly Correlated Multi-Orbital Systems : A Continuous-Time Quantum Monte Carlo Analysis}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-85253}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2013}, abstract = {In this thesis I present results concerning realistic calculations of correlated fermionic many-body systems. One of the main objectives of this work was the implementation of a hybridization expansion continuous-time quantum Monte Carlo (CT-HYB) algorithm and of a flexible self-consistency loop based on the dynamical mean-field theory (DMFT). DMFT enables us to treat strongly correlated electron systems numerically. After the implementation and extensive testing of the program we investigated different problems to answer open questions concerning correlated systems and their numerical treatment.}, subject = {Monte-Carlo-Simulation}, language = {en} } @phdthesis{Krauss2013, author = {Krauß, Martin Bernhard}, title = {Testing Models with Higher Dimensional Effective Interactions at the LHC and Dark Matter Experiments}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-94519}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2013}, abstract = {Dark matter and non-zero neutrino masses are possible hints for new physics beyond the Standard Model of particle physics. Such potential consequences of new physics can be described by effective field theories in a model independent way. It is possible that the dominant contribution to low-energy effects of new physics is generated by operators of dimension d>5, e.g., due to an additional symmetry. Since these are more suppressed than the usually discussed lower dimensional operators, they can lead to extremly weak interactions even if new physics appears at comparatively low scales. Thus neutrino mass models can be connected to TeV scale physics, for instance. The possible existence of TeV scale particles is interesting, since they can be potentially observed at collider experiments, such as the Large Hadron Collider. Hence, we first recapitulate the generation of neutrino masses by higher dimensional effective operators in a supersymmetric framework. In addition, we discuss processes that can be used to test these models at the Large Hadron Collider. The introduction of new particles can affect the running of gauge couplings. Hence, we study the compatibilty of these models with Grand Unified Theories. The required extension of these models can imply the existence of new heavy quarks, which requires the consideration of cosmological constraints. Finally, higher dimensional effective operators can not only generate small neutrino masses. They also can be used to discuss the interactions relevant for dark matter detection experiments. Thus we apply the methods established for the study of neutrino mass models to the systematic discussion of higher dimensional effective operators generating dark matter interactions.}, subject = {Neutrino}, language = {en} } @article{NejadLangHahnetal.2013, author = {Nejad, B. Chokoufe and Lang, J.-N. Lang and Hahn, T. and Mirabella, E.}, title = {FormCalc 8: Better Algebra and Vectorization}, series = {Acta Physica Polonica B}, volume = {44}, journal = {Acta Physica Polonica B}, number = {11}, doi = {10.5506/APhysPolB.44.2231}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-128618}, pages = {2231-2239}, year = {2013}, abstract = {We present Version 8 of the Feynman-diagram calculator FormCalc. New features include, in particular, significantly improved algebraic simplification as well as vectorization of the generated code. The Cuba Library, used in FormCalc, features checkpointing to disk for all integration algorithms.}, language = {en} } @phdthesis{Laubach2014, author = {Laubach, Manuel}, title = {Nichtmagnetische Isolatoren in Hexagonalen Gittermodellen}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-106987}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2014}, abstract = {Wir untersuchen zunächst das Hubbard-Modell des anisotropen Dreiecksgitters als effektive Beschreibung der Mott-Phase in verschiedenen organischen Verbindungen mit dreieckiger Gitterstruktur. Um die Eigenschaften am absoluten Nullpunkt zu bestimmen benutzen wir die variationelle Cluster Näherung (engl. variational cluster approximation VCA) und erhalten das Phasendiagramm als Funktion der Anisotropie und der Wechselwirkungsstärke. Wir finden f{\"u}r schwache Wechselwirkung ein Metall. F{\"u}r starke Wechselwirkung finden wir je nach Stärke der Anisotropie eine Néel oder eine 120◦-Néel antiferromagnetische Ordnung. In einem Bereich mittlerer Wechselwirkung entsteht in der Nähe des isotropen Dreiecksgitters ein nichtmagnetischer Isolator. Der Metall-Isolator-Übergang hängt maßgeblich von der Anisotropie ab, genauso wie die Art der magnetischen Ordnung und das Erscheinen und die Ausdehnung der nichtmagnetischen Isolatorphase. Spin-Bahn Kopplung ist der ausschlaggebende Parameter, der elektronische Bandmodelle in topologische Isolatoren wandelt. Spin-Bahn Kopplung im Allgemeinen beinhaltet auch den Rashba Term, der die SU(2) Symmetrie vollständig bricht. Sobald man auch Wechselwirkungen ber{\"u}cksichtigt, m{\"u}ssen sich viele theoretische Methoden auf die Analyse vereinfachter Modelle beschränken, die nur Spin-Bahn Kopplungen enthalten, welche die U(1) Symmetrie erhalten und damit eine Rashba Kopplung ausschließen. Wir versuchen diese bisher bestehende L{\"u}cke zu schließen und untersuchen das Kane-Mele Hubbard (KMH) Modell mit Rashba Spin-Bahn Kopplung und präsentieren eine systematische Analyse des Effekts der Rashba Spin-Bahn Kopplung in einem korrelierten zweidimensionalen topologischen Isolator. Wir wenden die VCA auf dieses Problem an und bestimmen das Phasendiagramm mit Wechselwirkung durch die Berechnung der lokalen Zustandsdichte, der Magnetisierung, der Einteilchenspektralfunktion und der Randzustände. Nach einer ausf{\"u}hrlichen Auswertung des KMH-Modells, bei erhaltener U(1) Symmetrie, finden wir auch f{\"u}r endliche Wechselwirkung, dass eine zusätzliche Rashba Kopplung zu neuen elektronischen Phasen f{\"u}hrt, wie eine metallische Phase und eine topologische Isolatorphase ohne Bandl{\"u}cke in der lokalen Zustandsdichte, die aber eine direkte Bandl{\"u}cke f{\"u}r jeden Wellenvektor besitzt. F{\"u}r eine Klasse von 5d Übergangsmetallen untersuchen wir ein KMH ähnliches Modell mit multidirektionaler Spin-Bahn Kopplung, das wegen seiner Relevanz f{\"u}r die Natrium-Iridate (engl. sodium iridate) als SI Modell bezeichnet wird. Diese intrinsische Kopplung bricht die SU(2) Symmetrie bereits vollständig und dennoch erhält man wegen der speziellen Form f{\"u}r starke Wechselwirkung wieder einen rotationssymmetrischen Néel-AFM Isolator. Der topologische Isolator des SIH-Modells ist adiabatisch mit dem des KMH-Modells verbunden, jedoch sind die Randströme hier nicht mehr spinpolarisiert. Wir verallgemeinern das Konzept der Klein-Transformation, das bereits erfolgreich auf Spin-Hamiltonians angewandt wurde, und wenden es auf ein Hubbard-Modell mit rein imaginären spinabhängigen H{\"u}pfen an, das im Grenzfall unendlicher Wechselwirkung in das Kitaev-Heisenberg Modell {\"u}bergeht. Dadurch erhält man ein Modell des Dreiecksgitters mit reellen spinunabhängigen H{\"u}pfen, das aber eine mehratomige Einheitszelle besitzt. F{\"u}r schwache Wechselwirkung ist das System ein Dirac Halbmetall und f{\"u}r starke Wechselwirkung erhält man eine 120◦-Néel antiferromagnetische Ordnung. F{\"u}r mittlere Wechselwirkung findet man aber einen relativ großen Bereich in dem eine nichtmagnetische Isolatorphase stabil ist. Unsere Ergebnisse deuten auf die mögliche Existenz einer Quanten Spinfl{\"u}ssigkeit hin.}, subject = {Hexagonaler Kristall}, language = {de} } @phdthesis{Janotta2014, author = {Janotta, Peter}, title = {Nonlocality and entanglement in Generalized Probabilistic Theories and beyond}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-105612}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2014}, abstract = {Quantum theory is considered to be the most fundamental and most accurate physical theory of today. Although quantum theory is conceptually difficult to understand, its mathematical structure is quite simple. What determines this particularly simple and elegant mathematical structure? In short: Why is quantum theory as it is? Addressing such questions is the aim of investigating the foundations of quantum theory. In the past this field of research was sometimes considered as an academic subject without much practical impact. However, with the emergence of quantum information theory this perception has changed significantly and both fields started to fruitfully influence each other. Today fundamental aspects of quantum theory attract increasing attention and the field belongs to the most exciting subjects of theoretical physics. This thesis is concerned with a particular branch in this field, namely, with so-called Generalized Probabilistic Theories (GPTs), which provide a unified theoretical framework in which classical and quantum theory emerge as special cases. This is used to examine nonlocal features that help to distinguish quantum theory from alternative toy theories. In order to extend the scope of theories that can be examined with the framework, we also introduce several generalizations to the framework itself. We start in Chapter 1 with introducing the standard GPT framework and summarize previous results, based on a review paper of the author [New J. Phys. 13, 063024 (2011)]. To keep the introduction accessible to a broad readership, we follow a constructive approach. Starting from few basic physically motivated assumptions we show how a given set of observations can be manifested in an operational theory. Furthermore, we characterize consistency conditions limiting the range of possible extensions. We point out that non-classical features of single systems can equivalently result from higher dimensional classical theories that have been restricted. Entanglement and non-locality, however, are shown to be genuine non-classical features. We review features that have been found to be specific for quantum theory separably or single and joint systems. Chapter 2 incorporates results published in [J. Phys. A 47(32), pp. 1-32 (2014)] and [Proc. QPL 2011 via EPTCS vol. 95, pp. 183-192 (2012)]. The GPT framework is applied to show how the structure of local state spaces indirectly affects possible nonlocal correlations, which are global properties of a theory. These correlations are stronger than those possible in a classical theory, but happen to show different restrictions that can be linked to the structure of subsystems. We first illustrate the phenomenon with toy theories with particular local state spaces. We than show that a particular class of joint states (inner product states), whose existence depends on geometrical properties of the local subsystems, can only have correlations for a known limited set called Q1. All bipartite correlations of both, quantum and classical correlations, can be mapped to measurement statistics from such joint states. Chapter 3 shows unpublished results on entanglement swapping in GPTs. This protocol, which is well known in quantum information theory, allows to nonlocally transfer entanglement to initially unentangled parties with the help of a third party that shares entanglement with each. We review our approach published in [Proc. QPL 2011 via EPTCS vol. 95, pp. 183-192 (2012)], which mimics the joint systems' structure of quantum theory by modifying a popular toy theory known as boxworld. However, it is illustrated that this approach fails for bigger multipartite systems due to inconsistencies evoked by entanglement swapping. It turns out that the GPT framework does not allow entanglement swapping for general subsystems with two-dimensional state spaces with transitive pure states. Altering the GPT framework to allow completely globally degrees of freedom, however, enables us to construct consistent entanglement swapping for these subsystems. This construction resembles the situation in quantum theory on a real Hilbert space. A questionable assumption usually taken in the standard GPT framework is the so-called no-restriction hypothesis. It states that the measurement that are possible in a theory can be derived from the state space. In fact, this assumption seems to exist for reasons of mathematical convenience, but it seems to lack physical motivation. We generalize the GPT framework to also account for systems that do not obey the no-restriction hypothesis in Chapter 4, which presents results published in [Phys. Rev. A 87, 052131 (2013)] and [Proc. QPL 2013, to be published in EPTCS]. The extended framework includes new classes of probabilistic theories. As an example, we show how to construct theories that include intrinsic noise. We also provide a "self-dualization" procedure that requires the violation of the no-restriction hypothesis. This procedure restricts the measurement of arbitrary theories such that the theories act as if they were self-dual. Self-duality has recently gathered lots of interest, since such theories share many features of quantum theory. For example Tsirelson's bound holds for correlations on the maximally entangled state in these theories. Finally, we characterize the maximal set of joint states that can be consistently defined for given subsystems. This generalizes the maximal tensor product of the standard GPT framework.}, subject = {Quantentheorie}, language = {en} } @article{CamargoMolinaGarbrechtO'Learyetal.2014, author = {Camargo-Molina, J. E. and Garbrecht, B. and O'Leary, B. and Porod, W. and Staub, F.}, title = {Constraining the Natural MSSM through tunneling to color-breaking vacua at zero and non-zero temperature}, series = {Physics Letters B}, volume = {737}, journal = {Physics Letters B}, doi = {10.1016/j.physletb.2014.08.036}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-118458}, pages = {156-161}, year = {2014}, abstract = {We re-evaluate the constraints on the parameter space of the minimal supersymmetric standard model from tunneling to charge- and/or color-breaking minima, taking into account thermal corrections. We pay particular attention to the region known as the Natural MSSM, where the masses of the scalar partners of the top quarks are within an order of magnitude or so of the electroweak scale. These constraints arise from the interaction between these scalar tops and the Higgs fields, which allows the possibility of parameter points having deep charge- and color-breaking true vacua. In addition to requiring that our electroweak-symmetry-breaking, yet QCD- and electromagnetism-preserving vacuum has a sufficiently long lifetime at zero temperature, also demanding stability against thermal tunneling further restricts the allowed parameter space.}, language = {en} } @phdthesis{Werner2014, author = {Werner, Jan}, title = {Numerical Simulations of Heavy Fermion Systems: From He-3 Bilayers to Topological Kondo Insulators}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-112039}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2014}, abstract = {Even though heavy fermion systems have been studied for a long time, a strong interest in heavy fermions persists to this day. While the basic principles of local moment formation, Kondo effect and formation of composite quasiparticles leading to a Fermi liquid, are under- stood, there remain many interesting open questions. A number of issues arise due to the interplay of heavy fermion physics with other phenomena like magnetism and superconduc- tivity. In this regard, experimental and theoretical investigations of He-3 can provide valuable insights. He-3 represents a unique realization of a quantum liquid. The fermionic nature of He-3 atoms, in conjunction with the absence of long-range Coulomb repulsion, makes this material an ideal model system to study Fermi liquid behavior. Bulk He-3 has been investigated for quite some time. More recently, it became possible to prepare and study layered He-3 systems, in particular single layers and bilayers. The pos- sibility of tuning various physical properties of the system by changing the density of He-3 and using different substrate materials makes layers of He-3 an ideal quantum simulator for investigating two-dimensional Fermi liquid phenomenology. In particular, bilayers of He-3 have recently been found to exhibit heavy fermion behavior. As a function of temperature, a crossover from an incoherent state with decoupled layers to a coherent Fermi liquid of composite quasiparticles was observed. This behavior has its roots in the hybridization of the two layers. The first is almost completely filled and subject to strong correlation effects, while the second layer is only partially filled and weakly correlated. The quasiparticles are formed due to the Kondo screening of localized moments in the first layer by the second-layer delocalized fermions, which takes place at a characteristic temperature scale, the coherence scale Tcoh. Tcoh can be tuned by changing the He-3 density. In particular, at a certain critical filling, the coherence scale is expected to vanish, corresponding to a divergence of the quasiparticle effective mass, and a breakdown of the Kondo effect at a quantum critical point. Beyond the critical point, the layers are decoupled. The first layer is a local moment magnet, while the second layer is an itinerant overlayer. However, already at a filling smaller than the critical value, preempting the critical point, the onset of a finite sample magnetization was observed. The character of this intervening phase remained unclear. Motivated by these experimental observations, in this thesis the results of model calcula- tions based on an extended Periodic Anderson Model are presented. The three particle ring exchange, which is the dominant magnetic exchange process in layered He-3, is included in the model. It leads to an effective ferromagnetic interaction between spins on neighboring sites. In addition, the model incorporates the constraint of no double occupancy by taking the limit of large local Coulomb repulsion. By means of Cellular DMFT, the model is investigated for a range of values of the chemical potential µ and inverse temperature β = 1/T . The method is a cluster extension to the Dy- namical Mean-Field Theory (DMFT), and allows to systematically include non-local correla- tions beyond the DMFT. The auxiliary cluster model is solved by a hybridization expansion CTQMC cluster solver, which provides unbiased, numerically exact results for the Green's function and other observables of interest. As a first step, the onset of Fermi liquid coherence is studied. At low enough temperature, the self-energy is found to exhibit a linear dependence on Matsubara frequency. Meanwhile, the spin susceptibility crossed over from a Curie-Weiss law to a Pauli law. Both observations serve as fingerprints of the Fermi liquid state. The heavy fermion state appears at a characteristic coherence scale Tcoh. This scale depends strongly on the density. While it is rather high for small filling, for larger filling Tcoh is increas- ingly suppressed. This involves a decreasing quasiparticle residue Z ∼ Tcoh and an enhanced mass renormalization m∗/m ∼ Tcoh-1. Extrapolation leads to a critical filling, where the co- herence scale is expected to vanish at a quantum critical point. At the same time, the effective mass diverges. This corresponds to a breakdown of the Kondo effect, which is responsible for the formation of quasiparticles, due to a vanishing of the effective hybridization between the layers. Taking only single-site DMFT results into account, the above scenario seems plausible. However, paramagnetic DMFT neglects the ring exchange interaction completely. In or- der to improve on this, Cellular DMFT simulations are conducted for small clusters of size Nc = 2 and 3. The results paint a different physical picture. The ring exchange, by favor- ing a ferromagnetic alignment of spins, competes with the Kondo screening. As a result, strong short-range ferromagnetic fluctuations appear at larger values of µ. By lowering the temperature, these fluctuations are enhanced at first. However, for T < Tcoh they are increas- ingly suppressed, which is consistent with Fermi liquid coherence. However, beyond a certain threshold value of µ, fluctuations persist to the lowest temperatures. At the same time, while not apparent in the DMFT results, the total occupation n increases quite strongly in a very narrow range around the same value of µ. The evolution of n with µ is always continuous, but hints at a discontinuity in the limit Nc → ∞. This first-order transition breaks the Kondo effect. Beyond the transition, a ferromagnetic state in the first layer is established, and the second layer becomes a decoupled overlayer. These observations provide a quite appealing interpretation of the experimental results. As a function of chemical potential, the Kondo breakdown quantum critical point is preempted by a first-order transition, where the layers decouple and the first layer turns into a ferromagnet. In the experimental situation, where the filling can be tuned directly, the discontinuous transition is mirrored by a phase separation, which interpolates between the Fermi liquid ground state at lower filling and the magnetic state at higher filling. This is precisely the range of the intervening phase found in the experiments, which is characterized by an onset of a finite sample magnetization. Besides the interplay of heavy fermion physics and magnetic exchange, recently the spin- orbit coupling, which is present in many heavy fermion materials, attracted a lot of interest. In the presence of time-reversal symmetry, due to spin-orbit coupling, there is the possibility of a topological ground state. It was recently conjectured that the energy scale of spin-orbit coupling can become dom- inant in heavy fermion materials, since the coherence scale and quasiparticle bandwidth are rather small. This can lead to a heavy fermion ground state with a nontrivial band topology; that is, a topological Kondo insulator (TKI). While being subject to strong correlation effects, this state must be adiabatically connected to a non-interacting, topological state. The idea of the topological ground state realized in prototypical Kondo insulators, in par- ticular SmB6, promises to shed light on some of the peculiarities of these materials, like a residual conductivity at the lowest temperatures, which have remained unresolved so far. In this work, a simple two-band model for two-dimensional topological Kondo insulators is devised, which is based on a single Kramer's doublet coupled to a single conduction band. The model is investigated in the presence of a Hubbard interaction as a function of interaction strength U and inverse temperature β. The bulk properties of the model are obtained by DMFT, with a hybridization expansion CTQMC impurity solver. The DMFT approximation of a local self-energy leads to a very simple way of computing the topological invariant. The results show that with increasing U the system can be driven through a topological phase transition. Interestingly, the transition is between distinct topological insulating states, namely the Γ-phase and M-phase. This appearance of different topological phases is possible due to the symmetry of the underlying square lattice. By adiabatically connecting both in- teracting states with the respective non-interacting state, it is shown that the transition indeed drives the system from the Γ-phase to the M-phase. A different behavior can be observed by pushing the bare position of the Kramer's doublet to higher binding energies. In this case, the non-interacting starting point has a trivial band topology. By switching on the interaction, the system can be tuned through a quantum phase transition, with a closing of the band gap. Upon reopening of the band gap, the system is in the Γ-phase, i. e. a topological insulator. By increasing the interaction strength further, the system moves into a strongly correlated regime. In fact, close to the expected transition to the M phase, the mass renormalization becomes quite substantial. While absent in the para- magnetic DMFT simulations conducted, it is conceivable that instead of a topological phase transition, the system undergoes a time-reversal symmetry breaking, magnetic transition. The regime of strong correlations is studied in more detail as a function of temperature, both in the bulk and with open boundary conditions. A quantity which proved very useful is the bulk topological invariant Ns, which can be generalized to finite interaction strength and temperature. In particular, it can be used to define a temperature scale T ∗ for the onset of the topological state. Rescaling the results for Ns, a nice data collapse of the results for different values of U, from the local moment regime to strongly mixed valence, is obtained. This hints at T ∗ being a universal low energy scale in topological Kondo insulators. Indeed, by comparing T ∗ with the coherence scale extracted from the self-energy mass renormalization, it is found that both scales are equivalent up to a constant prefactor. Hence, the scale T ∗ obtained from the temperature dependence of topological properties, can be used as an independent measure for Fermi liquid coherence. This is particularly useful in the experimentally relevant mixed valence regime, where charge fluctuations cannot be neglected. Here, a separation of the energy scales related to spin and charge fluctuations is not possible. The importance of charge fluctuations becomes evident in the extent of spectral weight transfer as the temperature is lowered. For mixed valence, while the hybridization gap emerges, a substantial amount of spectral weight is shifted from the vicinity of the Fermi level to the lower Hubbard band. In contrast, this effect is strongly suppressed in the local moment regime. In addition to the bulk properties, the spectral function for open boundaries is studied as a function of temperature, both in the local moment and mixed valence regime. This allows an investigation of the emergence of topological edge states with temperature. The method used here is the site-dependent DMFT, which is a generalization of the conventional DMFT to inhomogeneous systems. The hybridization expansion CTQMC algorithm is used as impurity solver. By comparison with the bulk results for the topological quantity Ns, it is found that the temperature scale for the appearance of the topological edge states is T ∗, both in the mixed valence and local moment regime.}, subject = {Fermionensystem}, language = {en} }