@phdthesis{Schreyer2015, author = {Schreyer, Manuel}, title = {Search for supersymmetry in events containing light leptons, jets and missing transverse momentum in \(\sqrt{s}\) = 8 TeV pp collisions with the ATLAS detector}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-120863}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2015}, abstract = {The results of two analyses searching for supersymmetry (SUSY) in data of the ATLAS experiment are presented in this thesis. The data were recorded in proton-proton collisions at the Large Hadron Collider in 2012 at a centre of mass energy of \(\sqrt{s}\)=8 TeV and correspond to an integrated luminosity of 20.3 fb\(^{-1}\). The first search is performed in signatures containing an opposite-sign electron or muon pair, which is compatible with originating from a Z boson decay, in addition to jets and large missing transverse momentum. The analysis targets the production of squarks and gluinos in R-parity conserving (RPC) models with SUSY breaking via General Gauge Mediation (GGM). The main Standard Model (SM) backgrounds are \(t\overline t\), WW, W+t and Z to \(\tau \tau\) processes which are entirely estimated from data using different-flavour events. Besides that, the SM production of Z bosons in association with jets and large fake missing momentum from mismeasurements plays a role and is predicted with the data-driven jet smearing method. Backgrounds from events with fake leptons are estimated with the data-driven matrix method. WZ/ZZ production as well as smaller background contributions are determined from Monte-Carlo simulations. The search observes an excess of data over the SM prediction with a local significance of 3.0 \(\sigma\) in the electron channel, 1.7 \(\sigma\) in the muon channel and 3.0 \(\sigma\) when the two channels are added together. The results are used to constrain the parameters of the GGM model. The second analysis uses the already published results of an ATLAS search for SUSY in events with one isolated electron or muon, jets and missing transverse momentum to reinterpret them in the context of squark and gluino production in SUSY models with R-parity violating (RPV) \(LQ\overline D\)-operators. In contrast to RPC models, the lightest SUSY particle (LSP) is not stable but decays into SM particles. "Standard" analyses often do not consider SUSY models with RPV although they are in principle sensitive to them. The exclusion limits on the squark and gluino mass obtained from the reinterpretation extend up to 1200 GeV. These are the first results by any ATLAS SUSY search which systematically cover a wide range of RPV couplings in the case of prompt LSP decays. However, the analysis is not sensitive to the full parameter space of the \(LQ\overline D\)-model and reveals gaps in the ATLAS SUSY program which have to be closed by dedicated search strategies in the future.}, subject = {Supersymmetrie}, language = {en} } @phdthesis{Meyer2012, author = {Meyer, Jochen}, title = {Muon performance aspects and measurement of the inclusive ZZ production cross section through the four lepton final state with the ATLAS experiment at the LHC}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-78793}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2012}, abstract = {The "Large Hadron Collider" (LHC) is currently the most powerful particle accelerator. It provides particle collisions at a center of mass energy in the Tera-electronvolt range, which had never been reached in a laboratory before. Thereby a new era in high energy particle physics has began. Now it is possible to test one of the most precise theories in physics, the Standard Model of particle physics, at these high energies. The purpose is particularly served by four large experiments installed at the LHC, namely "A Toroidal LHC ApparatuS" (ATLAS), the "Compact-Muon-Solenoid" (CMS), the "Large Hadron Collider beauty" (LHCb) and "A Large Ion Collider Experiment" (ALICE). Besides exploring the high energy behavior of the well-established portions of the Standard Model, one of the main objectives is to find the Higgs boson included in the model, but not discovered by any preceding effort. It is of tremendous importance since fermions and heavy electroweak gauge bosons acquire mass because of this boson. Although the success of the Standard Model in describing nature is already undisputed, there are some flaws due to observations inexplicable within this theory only. Therefore searches for physics beyond the Standard Model are promoted at the LHC experiments as well. In order to achieve the defined goals, crucial aspects are firstly precise measurements, to verify Standard Model predictions in detail, and secondly an evaluation of as much information as accessible by the detectors, to recognize new phenomena as soon as possible for subsequent optimizations. Both challenges are only possible with a superior understanding of the detectors. An inevitable contribution to attain this knowledge is a realistic simulation, partially requiring new implementation techniques to describe the very complex instrumentation. The research presented here is performed under the patronage of the ATLAS collaboration with a special focus on measurements done with muon spectrometer. Thus a first central issue is the performance of the spectrometer in terms of physics objects that are recognized by the device, the compatibility of data and the existing simulation as well as its improvement and finally the extension of the acceptance region. Once the excellent behavior and comprehension of the muon spectrometer is demonstrated, a second part addresses one physics use case of reconstructed muons. The electroweak force is part of the Standard Model and causes the interaction of heavy electroweak gauge bosons with fermions as well as their self-interaction. In proton-proton collisions such gauge bosons are produced. However, they decay immediately into a pair of fermions. In case of the Z boson, which is one of the gauge bosons, oppositely charged fermions of the same generation, including muons, emerge. The various decay modes are determined precisely at particle accelerators other than the LHC. However, the associated production of two Z bosons is measured less exactly at those facilities because of a very low cross section. The corresponding results acquired with the ATLAS experiment exceed all previous measurements in terms of statistics and accuracy. They are reported in this thesis as obtained from the observation of events with four charged leptons. The enhancement of the signal yield based on the extension of the muon spectrometer acceptance is especially emphasized as well as alternative methods to estimate background events. Furthermore, the impact on the probing of couplings of three Z bosons and intersection with the search for the Standard Model Higgs boson are pointed out.}, subject = {ATLAS }, language = {en} }