Supersymmetry with decoupled scalars and reconstruction and identification of electrons in the ATLAS detector

THe LHC is a proton collider with a center-of-mass enerby of 14 TeV located at CERN. First collisions are expected in fall 2009. The ATLAS experiment is, with CMS, one of two general-purpose experiments installed on the LHC. THe available energy and the high luminosity will allow ATLAS and CMS to search for the Higgs boson as well as other new particles predicted by models of physics beyond the Standard Model (SM) such as supersymmetry. Electrons are of major importance for the re-discovery of the SM as well as for the search for new physics. Also, they provide numerous indicators on the performance of the detector thanks to Z decays, bremsstrahlung effects and photon conversions. However, with 10^5 jets expected per signal electron, a severe rejection of the background is necessary. In this thesis, the reconstruction and identification of electrons in ATLAS is presented. A number of variables are studied in order to reject hadronic and electromagnetic background while optimizing signal efficiency. The performance of electron identification and of background rejection are estimated in a potentially unfavourable detection environment. Supersymmetry is an extension of the SM in which each particle of spin s is associated to a new spin |s-1/2| particle. The search for supersymmetry is one of the primary goals of ATLAS. In this thesis, the signatures of a supersymmetric model in which scalars are decoupled are studied. The discover y potential is quantified. Potential measurements, their background and uncertainties are investigated. These observables are then used to estimate the potential for the determination of the underlying parameters at the LHC. The Minimal Supersymmetric Standard Model is the most used model to study the phenomenology of supersymmetry at the LHC and future colliders. In this thesis, the precision on the determination of its parameters provided by two future linear e+e- colliders with respect to the LHC is estimated.