Réalisation du pré-échantillonneur central d'ATLAS et étude de la séparation $\gamma / \pi^0$ dans le calorimètre électromagnétique

ATLAS is one of the detectors which will equip the future proton-proton collider LHC at CERN. The main motivation for the ATLAS experiment is the quest for the Higgs boson. The observation of this particle would be an important step in the understending of particle physics in the context of the standard model, with or without supersymmetry. This thesis aims to present the construction of the barrel presampler which will equip the front face of the ATLAS electromagnetic calorimeter. The construction and tests of sectors were achieved at the Laboratory of Subatomic Physic and Cosmology of Grenoble. Two of these sectors were mounted on one module of the electromagnetic calorimeter and tested with electron, photon and muon beams at CERN. I participated in these tests and analysed the data. The results were compared to a Monte-Carlo simulation GEANT3. gamma / pi0 rejection was also studied. This is very important to evaluate the potentiel of the electromagnetic calorimeter to detect the Higgs boson in the decay channel H0 -> gamma gamma.