Performance du spectrometre a muons d’ALICE. Production et mesure des bosons faibles dans des collisions d’ions lourds aupres du LHC

Nucleus-nucleus collisions are the unique tool available to investigate the QCD matter phase diagram in the laboratory. A lot of work has been devoted to explore it in different domains in the last 3 decades, and the forthcoming LHC collider will contribute to such research increasing the collision center-of-mass energy by a factor 30 and the energy densities by a factor 1-10 with respect to the RHIC collider [C+04, A+06].Only a comprehensive analysis of a wide spectrum of experimental observables can help to fully characterize the prospected matter. In particular, valuable information is expected from charm and beauty production, the situation from SPS and RHIC charmonia data being puzzling [GdC07, Lei07, A+00a, Sco07], and the c¯c (b¯b) yields per central nucleon-nucleon collision being increased from 10 (0.1) to 110 (5) from RHIC to LHC [C+04, MG07]. Whether quarkonia will thermalize, will develop collective motion, will be further suppressed or regenerated are still open questions that LHC data might resolve. In addition, the amount of energy available in the center-of-mass will enable weak bosons production and measurement for the first time in heavy-ion collisions. In chapters 1 & 2 we succinctly expose this canvas, the theoretical basis and the previous experimental results are outlined emphasizing the interest of heavy quarks, quarkonia and weak bosons. In the first part, the ALICE detector which is a heavy-ion dedica ted experiment settled at the LHC [C+04] will be presented (chapter 3). Among other subdetectors, it disposes of a forward muon spectrometer that will allow to investigate the muon related probes (quarkonia, open beauty,...). This thesis work is devoted to exploit its performances to detect dimuons and high transverse momentum (pT ) muons in order to measure quarkonia, open-beauty and weak bosons production. The apparatus abilities to measure dimuons (explore quarkonia) and high-pT muons (investigate weak bosons) are discussed in chapter 4. There, factorization techniques are employed to unravel the different contributions to the efficiency; influences from the trigger algorithm, the reconstruction algorithm, the intrinsic chambers efficiency and the dead zones are identified and examined. This methodology promises to be useful to cross-check the goodness of the efficiency calculations with data. In the second part, weak bosons production in p-p, p-Pb and Pb-Pb collisions at 14, 8.8 and 5.5 TeV respectively are presented (chapter 5) [CMAF06, CdV07]. Some particularities of weak bosons, and the procedures used to generate them with the PYTHIA event generator are exposed. Their yields in the muonic decay channel are obtained in the whole pseudorapidity range as well as within the ALICE muon spectrometer acceptance. Special attention is payed to W production and decay charge asymmetries, as they can be exploited to sign their prod uction. In chapter 6 a compendium of the various sources contribution to the single muon spectra is presented. The reconstructed single muon distribution with the ALICE muon spectrometer for the different collision types are computed, and the expected statistics are estimated. The last part addresses the utility of weak bosons measurements in nucleus-nucleus collisions (chapter 7). The influence of the medium produced in the collisions on heavy quark and weak bosons production and on the muons in their decay are discussed. Predictions of the single muon suppression in a hot and dense matter are obtained with the help of a gluon-radiation formalism [DDCdVZ07, CdVDD+07]. The nuclear modification factor, the central-to-peripheral ratio and the muon yield ratios are examined. Finally, some possibilities to investigate in-medium effects on Z boson decay particles are mentioned.