[en] In a first theoretical part, this thesis presents the main results about the Z^o decays in heavy flavours, deduced from the Standard Model. Quark fragmentation are briefly developed. Semileptonic decays are explained and finally, the main models that are used to describe decay processes. In the experimental second part, ALEPH detector is described, electron identification means and methods, presented and the electromagnetic calorimeter calibration, studied. In the last part, the bidimensional distribution of momentum-transverse momentum of selected electrons is analysed and simulated. The monitoring of background noise and systematic errors are also presented. Finally, the physical results that have been obtained in this study, are given and discussed