@phdthesis{Krauss2015,
  author    = {Krauß, Manuel Ernst},
  title     = {Non-minimal supersymmetric models: LHC phenomenology and model discrimination},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-123555},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2015},
  abstract  = {It is generally agreed upon the fact that the Standard Model of particle physics can only be viewed as an effective theory that needs to be extended as it leaves some essential questions unanswered. The exact realization of the necessary extension is subject to discussion. Supersymmetry is among the most promising approaches to physics beyond the Standard Model as it can simultaneously solve the hierarchy problem and provide an explanation for the dark matter abundance in the universe. Despite further virtues like gauge coupling unification and radiative electroweak symmetry breaking, minimal supersymmetric models cannot be the ultimate answer to the open questions of the Standard Model as they still do not incorporate neutrino masses and are besides heavily constrained by LHC data. This does, however, not derogate the beauty of the concept of supersymmetry. It is therefore time to explore non-minimal supersymmetric models which are able to close these gaps, review their consistency, test them against experimental data and provide prospects for future experiments. The goal of this thesis is to contribute to this process by exploring an extraordinarily well motivated class of models which bases upon a left-right symmetric gauge group. While relaxing the tension with LHC data, those models automatically include the ingredients for neutrino masses. We start with a left-right supersymmetric model at the TeV scale in which scalar \(SU(2)_R\) triplets are responsible for the breaking of left-right symmetry as well as for the generation of neutrino masses. Although a tachyonic doubly-charged scalar is present at tree-level in this kind of models, we show by performing the first complete one-loop evaluation that it gains a real mass at the loop level. The constraints on the predicted additional charged gauge bosons are then evaluated using LHC data, and we find that we can explain small excesses in the data of which the current LHC run will reveal if they are actual new physics signals or just background fluctuations. In a careful evaluation of the loop-corrected scalar potential we then identify parameter regions in which the vacuum with the phenomenologically correct symmetry-breaking properties is stable. Conveniently, those regions favour low left-right symmetry breaking scales which are accessible at the LHC. In a slightly modified version of this model where a \(U(1)_R × U(1)_{B-L}\) gauge symmetry survives down to the TeV scale, we implement a minimal gauge-mediated supersymmetry breaking mechanism for which we calculate the boundary conditions in the presence of gauge kinetic mixing. We show how the presence of the extended gauge group raises the tree-level Higgs mass considerably so that the need for heavy supersymmetric spectra is relaxed. Taking the constraints from the Higgs sector into account, we then explore the LHC phenomenology of this model and point out where the expected collider signatures can be distinguished from standard scenarios. In particular if neutrino masses are explained by low-scale seesaw mechanisms as is done throughout this work, there are potentially spectacular signals at low-energy experiments which search for charged lepton flavour violation. The last part of this thesis is dedicated to the detailed exploration of processes like μ → e γ, μ → 3 e or μ-e conversion in nuclei in a supersymmetric framework with an inverse seesaw mechanism. In particular, we disprove claims about a non-decoupling effect in Z-mediated three-body decays and study the prospects for discovering and distinguishing signals at near-future experiments. In this context we identify the possibility to deduce from ratios like BR(\(τ → 3 μ\))/BR(\(τ → μ e^+ e^-\)) whether the contributions from ν - W loops dominate over supersymmetric contributions or vice versa.},
  subject      = {Supersymmetrie},
  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}
}