Abstract : Grand unification theories are a possible extension of the standard
model. These theories imply the existence of an extra gauge boson, Z0.
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 Z0 decaying into lepton
pairs (mainly in electron pairs). A study of the kinematic and of the background
has been firstly performed. This analysis deals with the possibility
to distinguish different grand unification models, and with the possibility to
discover the Z0 boson, for masses from 1 TeV up to 4 TeV.