@phdthesis{Geissler2017, author = {Geißler, Florian}, title = {Transport properties of helical Luttinger liquids}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-153450}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2017}, abstract = {The prediction and the experimental discovery of topological insulators has set the stage for a novel type of electronic devices. In contrast to conventional metals or semiconductors, this new class of materials exhibits peculiar transport properties at the sample surface, as conduction channels emerge at the topological boundaries of the system. In specific materials with strong spin-orbit coupling, a particular form of a two-dimensional topological insulator, the quantum spin Hall state, can be observed. Here, the respective one-dimensional edge channels are helical in nature, meaning that there is a locking of the spin orientation of an electron and its direction of motion. Due to the symmetry of time-reversal, elastic backscattering off interspersed impurities is suppressed in such a helical system, and transport is approximately ballistic. This allows in principle for the realization of novel energy-efficient devices, ``spintronic`` applications, or the formation of exotic bound states with non-Abelian statistics, which could be used for quantum computing. The present work is concerned with the general transport properties of one-dimensional helical states. Beyond the topological protection mentioned above, inelastic backscattering can arise from various microscopic sources, of which the most prominent ones will be discussed in this Thesis. As it is characteristic for one-dimensional systems, the role of electron-electron interactions can be of major importance in this context. First, we review well-established techniques of many-body physics in one dimension such as perturbative renormalization group analysis, (Abelian) bosonization, and Luttinger liquid theory. The latter allow us to treat electron interactions in an exact way. Those methods then are employed to derive the corrections to the conductance in a helical transport channel, that arise from various types of perturbations. Particularly, we focus on the interplay of Rashba spin-orbit coupling and electron interactions as a source of inelastic single-particle and two-particle backscattering. It is demonstrated, that microscopic details of the system, such as the existence of a momentum cutoff, that restricts the energy spectrum, or the presence of non-interacting leads attached to the system, can fundamentally alter the transport signature. By comparison of the predicted corrections to the conductance to a transport experiment, one can gain insight about the microscopic processes and the structure of a quantum spin Hall sample. Another important mechanism we analyze is backscattering induced by magnetic moments. Those findings provide an alternative interpretation of recent transport measurements in InAs/GaSb quantum wells.}, subject = {Topologischer Isolator}, language = {en} } @article{ErdmengerFernandezFloryetal.2017, author = {Erdmenger, Johanna and Fern{\´a}ndez, Daniel and Flory, Mario and Meg{\´i}as, Eugenio and Straub, Ann-Kathrin and Witkowski, Piotr}, title = {Time evolution of entanglement for holographic steady state formation}, series = {Journal of High Energy Physics}, volume = {2017}, journal = {Journal of High Energy Physics}, number = {10}, doi = {10.1007/JHEP10(2017)034}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-173798}, year = {2017}, abstract = {Within gauge/gravity duality, we consider the local quench-like time evolution obtained by joining two 1+1-dimensional heat baths at different temperatures at time \(t\) = 0. A steady state forms and expands in space. For the 2+1-dimensional gravity dual, we find that the "shockwaves" expanding the steady-state region are of spacelike nature in the bulk despite being null at the boundary. However, they do not transport information. Moreover, by adapting the time-dependent Hubeny-Rangamani-Takayanagi prescription, we holographically calculate the entanglement entropy and also the mutual information for different entangling regions. For general temperatures, we find that the entanglement entropy increase rate satisfies the same bound as in the 'entanglement tsunami' setups. For small temperatures of the two baths, we derive an analytical formula for the time dependence of the entanglement entropy. This replaces the entanglement tsunami-like behaviour seen for high temperatures. Finally, we check that strong subadditivity holds in this time-dependent system, as well as further more general entanglement inequalities for five or more regions recently derived for the static case.}, language = {en} } @article{TemmeAdamAhnenetal.2017, author = {Temme, Fabian and Adam, Jan and Ahnen, Max L. and Baack, Dominik and Balbo, Matteo and Bergmann, Matthias and Biland, Adrian and Blank, Michael and Bretz, Thomas and Br{\"u}gge, Kai A. and Buss, Jens and Dmytriiev, Anton and Dorner, Daniela and Einecke, Sabrina and Hempfling, Christina and Hildebrand, Dorothee and Hughes, Gareth and Linhoff, Lena and Mannheim, Karl and M{\"u}ller, Sebastian and Neise, Dominik and Neronov, Andrii and N{\"o}the, Max and Paravac, Aleksander and Pauss, Felicitas and Rhode, Wolfgang and Shukla, Amit and Thaele, Julia and Walter, Roland}, title = {Long-Term monitoring of bright blazars in the multi-GeV to TeV range with FACT}, series = {Galaxies}, volume = {5}, journal = {Galaxies}, number = {1}, publisher = {MDPI}, issn = {2075-4434}, doi = {10.3390/galaxies5010018}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-198088}, pages = {18}, year = {2017}, abstract = {Blazars like Markarian 421 or Markarian 501 are active galactic nuclei (AGN), with their jets orientated towards the observer. They are among the brightest objects in the very high energy (VHE) gamma ray regime (>100 GeV). Their emitted gamma-ray fluxes are extremely variable, with changing activity levels on timescales between minutes, months, and even years. Several questions are part of the current research, such as the question of the emission regions or the engine of the AGN and the particle acceleration. A dedicated longterm monitoring program is necessary to investigate the properties of blazars in detail. A densely sampled and unbiased light curve allows for observation of both high and low states of the sources, and the combination with multi-wavelength observation could contribute to the answer of several questions mentioned above. FACT (First G-APD Cherenkov Telescope) is the first operational telescope using silicon photomultiplier (SiPM, also known as Geigermode—Avalanche Photo Diode, G-APD) as photon detectors. SiPM have a very homogenous and stable longterm performance, and allow operation even during full moon without any filter, leading to a maximal duty cycle for an Imaging Air Cherenkov Telescope (IACT). Hence, FACT is an ideal device for such a longterm monitoring of bright blazars. A small set of sources (e.g., Markarian 421, Markarian 501, 1ES 1959+650, and 1ES 2344+51.4) is currently being monitored. In this contribution, the FACT telescope and the concept of longterm monitoring of bright blazars will be introduced. The results of the monitoring program will be shown, and the advantages of densely sampled and unbiased light curves will be discussed.}, language = {en} } @article{BistiRogalevKarolaketal.2017, author = {Bisti, F. and Rogalev, V. A. and Karolak, M. and Paul, S. and Gupta, A. and Schmitt, T. and G{\"u}ntherodt, G. and Eyert, V. and Sangiovanni, G. and Profeta, G. and Strocov, V. N.}, title = {Weakly-correlated nature of ferromagnetism in nonsymmorphic CrO\(_2\) revealed by bulk-sensitive soft-X-ray ARPES}, series = {Physical Review X}, volume = {7}, journal = {Physical Review X}, number = {4}, doi = {10.1103/PhysRevX.7.041067}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-172251}, year = {2017}, abstract = {Chromium dioxide CrO\(_2\) belongs to a class of materials called ferromagnetic half-metals, whose peculiar aspect is that they act as a metal in one spin orientation and as a semiconductor or insulator in the opposite one. Despite numerous experimental and theoretical studies motivated by technologically important applications of this material in spintronics, its fundamental properties such as momentumresolved electron dispersions and the Fermi surface have so far remained experimentally inaccessible because of metastability of its surface, which instantly reduces to amorphous Cr\(_2\)O\(_3\). In this work, we demonstrate that direct access to the native electronic structure of CrO\(_2\) can be achieved with soft-x-ray angle-resolved photoemission spectroscopy whose large probing depth penetrates through the Cr\(_2\)O\(_3\) layer. For the first time, the electronic dispersions and Fermi surface of CrO\(_2\) are measured, which are fundamental prerequisites to solve the long debate on the nature of electronic correlations in this material. Since density functional theory augmented by a relatively weak local Coulomb repulsion gives an exhaustive description of our spectroscopic data, we rule out strong-coupling theories of CrO\(_2\). Crucial for the correct interpretation of our experimental data in terms of the valence-band dispersions is the understanding of a nontrivial spectral response of CrO\(_2\) caused by interference effects in the photoemission process originating from the nonsymmorphic space group of the rutile crystal structure of CrO\(_2\).}, language = {en} } @article{ChiesaGreinerSchoenherretal.2017, author = {Chiesa, Mauro and Greiner, Nicolas and Sch{\"o}nherr, Marek and Tramontano, Francesco}, title = {Electroweak corrections to diphoton plus jets}, series = {Journal of High Energy Physics}, journal = {Journal of High Energy Physics}, number = {10}, doi = {10.1007/JHEP10(2017)181}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-173512}, year = {2017}, abstract = {We calculate the next-to-leading order electroweak corrections to the production of a photon pair in association with zero, one and two jets at the LHC. We use GoSam and Sherpa to obtain the results in a fully automated way. For a typical set of fiducial cuts the electroweak corrections lead to a modification of the total cross section of up to 3\%, depending on the jet multiplicity. We find substantial contributions in differential distributions, leading to tens of per cent corrections for phase space regions within the reach of the LHC. Furthermore we investigate the importance of photon induced processes as well as subleading contributions. Photon induced processes are found to be negligible, subleading contributions can have a sizeable impact however they can be removed by appropriate phase space cuts.}, language = {en} } @article{DennerLangPellenetal.2017, author = {Denner, Ansgar and Lang, Jean-Nicolas and Pellen, Mathieu and Uccirati, Sandro}, title = {Higgs production in association with off-shell top-antitop pairs at NLO EW and QCD at the LHC}, series = {Journal of High Energy Physics}, journal = {Journal of High Energy Physics}, number = {2}, doi = {10.1007/JHEP02(2017)053}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-171871}, year = {2017}, abstract = {We present NLO electroweak corrections to Higgs production in association with off-shell top-antitop quark pairs. The full process pp → e +νeµ -ν¯µbb¯H is considered, and hence all interference, off-shell, and non-resonant contributions are taken into account. The electroweak corrections turn out to be below one per cent for the integrated cross section but can exceed 10\% in certain phase-space regions. In addition to its phenomenological relevance, the computation constitutes a major technical achievement as the full NLO virtual corrections involving up to 9-point functions have been computed exactly. The results of the full computation are supported by two calculations in the double-pole approximation. These also allow to infer the effect of off-shell contributions and emphasise their importance especially for the run II of the LHC. Finally, we present combined predictions featuring both NLO electroweak and QCD corrections in a common set-up that will help the experimental collaborations in their quest of precisely measuring the aforementioned process.}, language = {en} } @article{LindertPozzoriniBoughezaletal.2017, author = {Lindert, J. M. and Pozzorini, S. and Boughezal, R. and Campbell, J. M. and Denner, A. and Dittmaier, S. and Gehrmann-De Ridder, A. and Gehrmann, T. and Glover, N. and Huss, A. and Kallweit, S. and Maierh{\"o}fer, P. and Mangano, M. L. and Morgan, T. A. and M{\"u}ck, A. and Petriello, F. and Salam, G. P. and Sch{\"o}nherr, M. and Williams, C.}, title = {Precise predictions for \(V+\)jets dark matter backgrounds}, series = {European Physical Journal C}, volume = {77}, journal = {European Physical Journal C}, doi = {10.1140/epjc/s10052-017-5389-1}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-172555}, year = {2017}, abstract = {High-energy jets recoiling against missing transverse energy (MET) are powerful probes of dark matter at the LHC. Searches based on large MET signatures require a precise control of the \({Z(ν\overline{ν})}+\) jet background in the signal region. This can be achieved by taking accurate data in control regions dominated by \(Z(ℓ^+ℓ^-)+\) jet, \(W(ℓν)+\) jet and \(γ+\) jet production, and extrapolating to the \({Z(ν\overline{ν})}+\) jet background by means of precise theoretical predictions. In this context, recent advances in perturbative calculations open the door to significant sensitivity improvements in dark matter searches. In this spirit, we present a combination of state-of-the-art calculations for all relevant \(V+\) jets processes, including throughout NNLO QCD corrections and NLO electroweak corrections supplemented by Sudakov logarithms at two loops. Predictions at parton level are provided together with detailed recommendations for their usage in experimental analyses based on the reweighting of Monte Carlo samples. Particular attention is devoted to the estimate of theoretical uncertainties in the framework of dark matter searches, where subtle aspects such as correlations across different \(V+\) jet processes play a key role. The anticipated theoretical uncertainty in the \({Z(ν\overline{ν})}+\) jet background is at the few percent level up to the TeV range.}, language = {en} } @article{BiedermannDennerDittmaieretal.2017, author = {Biedermann, Benedikt and Denner, Ansgar and Dittmaier, Stefan and Hofer, Lars and J{\"a}ger, Barbara}, title = {Next-to-leading-order electroweak corrections to the production of four charged leptons at the LHC}, series = {Journal of High Energy Physics}, journal = {Journal of High Energy Physics}, number = {1}, doi = {10.1007/JHEP01(2017)033}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-171966}, year = {2017}, abstract = {We present a state-of-the-art calculation of the next-to leading-order electroweak corrections to ZZ production, including the leptonic decays of the Z bosons into μ\(^+\)μ\(^ -\)e\(^+\)e\(^-\) or μ\(^+\)μ\(^-\)μ\(^+\)μ\(^-\) final states. We use complete leading-order and next-to-leading-order matrix elements for four-lepton production, including contributions of virtual photons and all off-shell effects of Z bosons, where the finite Z-boson width is taken into account using the complex-mass scheme. The matrix elements are implemented into Monte Carlo programs allowing for the evaluation of arbitrary differential distributions. We present integrated and differential cross sections for the LHC at 13 TeV both for an inclusive setup where only lepton identification cuts are applied, and for a setup motivated by Higgs-boson analyses in the four-lepton decay channel. The electroweak corrections are divided into photonic and purely weak contributions. The former show the well-known pronounced tails near kinematical thresholds and resonances; the latter are generically at the level of ∼ -5\% and reach several -10\% in the high-energy tails of distributions. Comparing the results for μ\(^+\)μ\(^-\)e\(^+\)e\(^-\) and μ\(^+\)μ\(^-\)μ\(^+\)μ\(^-\) final states, we find significant differences mainly in distributions that are sensitive to the μ\(^+\)μ\(^-\) pairing in the μ\(^+\)μ\(^-\)μ\(^+\)μ\(^-\) final state. Differences between μ\(^+\)μ\(^-\)e\(^+\)e\(^-\) and μ\(^+\)μ\(^-\)μ\(^+\)μ\(^-\) channels due to interferences of equal-flavour leptons in the final state can reach up to 10\% in off-shell-sensitive regions. Contributions induced by incoming photons, i.e. photon-photon and quark-photon channels, are included, but turn out to be phenomenologically unimportant.}, language = {en} } @article{OPUS4-17217, title = {All-sky search for high-energy neutrinos from gravitational wave event GW170104 with the ANTARES neutrino telescope}, series = {European Physical Journal C}, volume = {77}, journal = {European Physical Journal C}, organization = {The ANTARES Collaboration}, doi = {10.1140/epjc/s10052-017-5451-z}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-172174}, year = {2017}, abstract = {Advanced LIGO detected a significant gravitational wave signal (GW170104) originating from the coalescence of two black holes during the second observation run on January 4th, 2017. An all-sky high-energy neutrino follow-up search has been made using data from the Antares neutrino telescope, including both upgoing and downgoing events in two separate analyses. No neutrino candidates were found within ±500 s around the GW event time nor any time clustering of events over an extended time window of ±3 months. The non-detection is used to constrain isotropic-equivalent high-energy neutrino emission from GW170104 to less than ∼ 1.2 × \(10^{55}\) erg for a \(E^{-2}\) spectrum. This constraint is valid in the energy range corresponding to the 5-95\% quantiles of the neutrino flux [3.2 TeV; 3.6 PeV], if the GW emitter was below the Antares horizon at the alert time.}, language = {en} } @article{CernaVelazcoFaberJonesPerezetal.2017, author = {Cerna-Velazco, Nhell and Faber, Thomas and Jones-P{\´e}rez, Joel and Porod, Werner}, title = {Constraining sleptons at the LHC in a supersymmetric low-scale seesaw scenario}, series = {European Physical Journal C}, volume = {77}, journal = {European Physical Journal C}, doi = {10.1140/epjc/s10052-017-5231-9}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-173809}, year = {2017}, abstract = {We consider a scenario inspired by natural supersymmetry, where neutrino data is explained within a low-scale seesaw scenario. We extend the Minimal Supersymmetric Standard Model by adding light right-handed neutrinos and their superpartners, the R-sneutrinos, and consider the lightest neutralinos to be higgsino-like. We consider the possibilities of having either an R-sneutrino or a higgsino as lightest supersymmetric particle. Assuming that squarks and gauginos are heavy, we systematically evaluate the bounds on slepton masses due to existing LHC data.}, language = {en} }