Etude des performances du calorimètre électromagnétique tonneau d'ATLAS. Mesure de l'asymétrie avant-arrière dans les événements $q\bar{q}\rightarrow Z/\gamma^* \rightarrow e^+e^-$

The start up of the ATLAS experiment at the CERN LHC is planned for the year 2007. The physics program of the experiment covers a wide field, going from tests of Standard Model (Higgs boson discovery) to new theories beyond the SM (Supersymmetry, extra dimensions ... etc). The work presented in this thesis returns within the framework of preparation to the starting of this experiment. After having presented the 2004 combined run, its installation, pedestal data and calibration data analysis, we develop a method for calibrating the energy measurement based on Geant4 Monte Carlo simulation of the combined run. These simulations are done in the general framework developed for the analysis of the ATLAS data. We present then the performance studies of the electromagnetic calorimeter as well as the results obtained: a sampling term of the energy resolution of 10.6%GeV$^{0.5}$ and local constant term of 0.43%, a non-uniformity of response of 0.44% giving a total constant term of 0.6% and a linearity better than 0.2% for electrons energies between 20 and 250 GeV. Concerning the "physics" side of this thesis, we show a first study on the determination of the effective weak mixing angle, $\sin^{2} \theta^{lept}_{eff}$ with one precision better than the current results, $\sim$10$^{-4}$. To reach such a precision it will be necessary to identify the electrons in the forward regions of the detector. This point is the subject of the last part of this manuscrit, it shows that one can reach an electron-jet rejection of 100 with an efficiency of the electrons reconstruction of 50%, by using a discriminating analysis based on the methods of Fisher, the likelihood and the neural networks.