[en] Grand unification theories are a possible extension of the standard model. These theories imply the existence of an extra gauge boson: Z'. The study of the parameters of this boson in its electronic decays, especially its width, needs a good uniformity and a good resolution of the electromagnetic calorimeter. The first part of this thesis is dedicated to the study of the response of the barrel of the electromagnetic calorimeter to a 245 GeV electron beam. The major part of this work was the measurement of the uniformity. To perform this analysis the search of problematic channel has been made and corrections have been developed to take into account effect from the detector and to use all the recorded events. In the second part, I have studied the potential of Atlas in the study of the Z' decaying into lepton pairs (mainly in electron pairs). A study of the kinematic and of the background has been performed. This analysis deals with the possibility to distinguish different grand unification models, and with the possibility to discover the Z' boson, for masses from 1 TeV up to 4 TeV. (author)