Search for supersymmetry events with two same-sign leptons

Supersymmetry is a hypothetic symmetry between bosons and fermions, which is broken by an unknown mechanism. So far, there is no experimental evidence for the existence of supersymmetric particles. Some Supersymmetry scenarios are predicted to be within reach of the ATLAS detector at the Large Hadron Col- lider. Final states with two isolated leptons (muons and electrons), that have same signs of charge, are suitable for the discovery of supersymmetric cascade decays. There are numerous supersymmetric processes that can yield final states with two same-sign or more leptons. Typically, these processes tend to have long cas- cade decay chains, producing high-energetic jets. Charged leptons are produced from decaying charginos and neutralinos in the cascades. If the R-parity is con- served and the lightest supersymmetric particle is a neutralino, supersymmetric processes lead to a large amount of missing energy in the detector. The most important Standard Model background for the same-sign dilepton channel is the semileptonic decay of top-antitop-pairs. One lepton originates from the leptonic decay of the W boson, the other lepton originates from one of the b quarks. Here, the neutrinos are responsible for the missing energy. The Standard Model background can be strongly reduced by applying cuts on the transverse momenta of jets, on the missing energy and on the lepton isolation. In this thesis, a cut-based analysis is presented, which is suitable for a discovery of the ATLAS SU4 point and other supersymmetric models in the mSUGRA parameter space. The analysis is optimized for a LHC center-of-mass energy of 10 TeV and for an integrated luminosity of 200 pb − 1 . A method to improve the estimation of the QCD background is presented and taken into account in the analysis. Furthermore, a method to estimate the Standard Model background from data is presented. With an extensive statistical analysis, the discovery- and the exclusion potential is discussed for the ATLAS SU4 point and several additional example points in the mSUGRA parameter space.