Identification des électrons dans la partie avant du calorimètre électromagnétique d'ATLAS au LHC et analyse des premières données

The start up of the ATLAS experiment at the CERN LHC has been done during the automn 2009. During the construction and integration of the detector, combined beam tests grouping several subsystems have been carried out. In the forward region of the detector (eta> 2.5), a combined beam test with electromagnetic and hadronic calorimeters has been done, whose data (pions and electrons) has been analyzed. Identification of electrons in this region can be used to study decays of Z and W bosons and also to develop some tools to understand the bakground noises. A method to estimate rejection of pions and electrons identification efficiency is presented using a discriminant analysis based on the methods of Fisher discriminant and on Boosted Decision Trees. It is shown that a pion rejection higher than 200 with an efficiency of electron identification of 50% can be obtained. Moreover the tools and methods developed during the beam tests have been applied on the first data of the LHC with collisions at 7 TeV. Since the present luminosity of the LHC is not yet sufficient to study precisely production of Z and W bosons by using data, a study using the Pythia generator has been done on electrons physics in the forward region