Étude des décroissances semi-muoniques de saveurs lourdes à bas Pt, et de l'efficacité de trajectographie du spectromètre à muons d'ALICE

In accelerators as the one of LHC, collisions will help to reproduce the Quark Gluon Plasma. With such a purpose, the ALICE detector is optimized to study the transition toward this hypothetic state of matter. The ALICE's muon spectrometer will measure the muon related probes (quarkonia, heavy flavours...). The first part of this thesis presents the method to calculate the tracking efficiency of this spectrometer. The results, obtained by simulation, are placed in the global efficiency context. They show the evolution of the efficiency as a function of likely electronics failures. They establish that a proper working of the detector involves less than 90% of channels failures and 85% of missing so-called MANU cards. The second part presents the Distance Closest Approach method which enhance the identification of muons from charm and beauty particle decays. For momentum as for pt < 4 GeV/c, the determination of heavy flavours contribution (charm in particular) in the single muon spectrum requires subtracting the decay part from pions and kaons. This discrimination is not possible track by track, an alternative method to subtract the light-hadrons has been developed. The DCA method uses the distance between the extrapolated track in the primary vertex transverse plan and the vertex itself. The different shapes of the distributions in DCA between the signal and the noise, arising from the different decay length path to the particle types, permit a better separation, and therefore a better estimation of the corresponding cross sections.