Mise en service du calorimetre electromagnetique d'Atlas et determination du potentiel de decouverte d'un Z' --> e+e- dans les premieres donnees LHC

After about fifteen years of development, the ATLAS detector is ready to operate and recorded, in 2008, several millions of cosmic events as well as first LHC data. This achievement is based on the long experience of beam tests and on the large effort towards the detector in situ commissioning undertaken by the ATLAS collaboration. This promises fast ability to perform searches for evidence of Higgs boson and new physics. I heavily contributed to the in situ commissioning of the EM calorimeter. To verify its performance, I studied the first cosmic data taken in 2006 which allowed the first in situ analysis of dead channels, energy reconstruction and detector response uniformity. This participation to the commissioning has continued with the study of the single beam data recorded during the first week of LHC operation (Sept. 2008). Expanding on my expertise of the EM calorimeter, I focused my physics analysis, prepared with simulation, on the promising discovery potential of new physics at LHC via the di-electron/di-photon decay of new heavy gauge boson in the early LHC data (the first 100 pb-1). Possible limitations coming from early hardware problems or imperfect electron energy calibration in first data have been estimated. According to the new schedule of LHC operation, this analysis will be possible with 10 TeV pp collisions data in 2010.