Identification of hadronic τ decays and observation potentional of CP-violating effects in SUSY at ATLAS

In November 2009 the ATLAS experiment started operation at the Large Hadron Collider (LHC) at CERN. The detector is optimized to search for the Higgs Boson and new physics at the TeV scale. Until the end of the data-taking period with proton-proton collisions on November 3rd, 2010, the ATLAS detector recorded an integrated luminosity of 45.0 pb-1 at a center-of-mass energy of √(s) = 7 TeV. In many signals of the Standard Model and new physics (e.g. SUSY and Higgs) τ-leptons play an important role. A cut-based approach for the identification of hadronically decaying τ-leptons is being used, particularly for the first data-taking period. Using Monte Carlo Data, the development of a cut-based identification method for hadronically decaying τ-lepton with the ATLAS detector at the Large Hadron Collider (LHC) with a center-of-mass energy of √(s) = 14 TeV is presented. The separation of signal and the large QCD jet background is a challenge to the identification of hadronically decaying τ-lepton. The identification is separated into two methods: the calorimeter-based method uses exclusive calorimeter information, while the calorimeter+track-based method combines calorimeter and tracking information. The cut optimization is separately accomplished for τ candidates with one charged decay product (1-prong) and τ candidates with three charged decay products (3-prong). Additionally the optimisation is split into bins of the visible transverse energy of the τ candidate (ETvis). First of all the optimization is presented and afterwards the performance of the cut-based identification method is discussed. The reconstruction efficiency for τ-leptons is determined by comparing first data corresponding to an integrated luminosity of 244 nb-1 and Monte Carlo simulation. The effect of systematic uncertainties is investigated. The CP violation predicted by the Standard Model is not sufficient to explain the matter - anti-matter asymmetry in the universe of the order of O(10-10). Hence new sources of CP violation are required. One possible approach is CP violation in the supersymmetric extension of the Standard Model. The CP violation can be evoked in models with so-called 'minimal supergravity' (mSUGRA) by introducing a complex phase. In such models the lightest supersymmetric particle (LSP) is the neutralino one. The triple product of the final state in cascade decay chains with two- or three-body decays is sensitive for CP-violating effects. For this purpose the mass and momentum of all decay products must be known. The potential to observe CP violation in t cascade decay chains with the ATLAS detector is investigated, with the CP violation added to the trilinear coupling φA. The momentum reconstruction of the neutralino one as well as the determination of the triple product asymmetry are presented. (orig.)