Etude de la désintégration du boson de Higgs dans le canal en quatre électrons dans le détecteur ATLAS

This thesis aims to study the decay of the Higgs boson in the H → Z Z^ (∗) → 4e channel in the ATLAS detector. This study began when the LHC was not in operation and therefore focused mainly on simulations. It became soon clear that the main issue was the performance of the electron reconstruction and identification, as the reconstruction efficiency of the Higgs boson in this channel depends on the 4th power of the electron reconstruction efficiency. This work therefore focuses on the study of the electron reconstruction, first on assessing few impacts of the bremsstrahlung effect, then the reconstruction improvement, and the identification optimization. Optimization of electron identification cuts has been done to make them more robust and consistent with the trigger system in anticipation of the first data from LHC at 7 TeV. This enabled to get a betteridentification efficiency while maintaining the necessary rejection of backgrounds (hadrons, heavy quark, photon conversions). To improve further the reconstruction and identification, new variables have been proposed for the association between the trace and the electromagnetic cluster, less sensitive to the bremmstrahlung effect and discriminant with respect to the background. In parallel, the impact of those improvements on the H → Z Z^(∗) → 4e analysis has been studied. It has also been proposed to modify the selection cuts for the events with four electrons so that the signal over background ratio gets improving. Finally, the LHC startup has allowed to perform the first study of real electrons from minimum bias events and from Z bosons decays. The studies of the characteristic variables (shower, track) have shown discrepancies with respect to the Monte-Carlo predictions, and indicated misalignments effect between the the electromagnetic calorimeter and the inner tracker. In the same time, with the data accumulating, the first events Z → ee with additionnal electrons have been observed, which inaugurates the Higgs background studies era.