Préparation de la mesure de la section efficace inclusive du $Z \to e^{+}e^{-}$ dans ATLAS. Etude des premières données avec le calorimètre électromagnétique.

Proton-proton collisions at 7 TeV in the center-of-mass at the LHC will produce many Z-bosons for which the measurement of the production cross-section is a test of the Standard Model. After a description of the theoretical context and of the ATLAS detector, simulation and analysis tools, a part of this thesis is dedicated to the use of cosmic rays events recorded during the commissioning in order to validate the simulation of the electromagnetic shower shapes. A second part gives a description of the keys steps for the cross-section measurements. Selection criteria based on isolation variables -alternatively to the standard selection- have been studied and used to observe the first Z->ee in ATLAS. Finally, the main sources of systematic errors implied in the cross-section measurement have been studied, specifically those linked to the electron selection efficiencies and the geometrical acceptance of the detector.