Misure di open charm in collisioni Pb-Pb nell'esperimento ALICE

LHC is the Large Hadron Collider situated in Geneva (Switzerland) where scientists from all over the world can study nuclear and particle physics. ALICE (A Large Ion Collider Experiment) is one of the four main experiments at the LHC, the only one optimized for the study of heavy-ion collisions at a centre-of-mass energy of 5.5 TeV per nucleon. The aim of the ALICE experiment is to study the behaviour of nuclear matter at very high temperatures and densities, that can be reached in Pb-Pb central collisions. In particular, one of the purpose of this experiment is to study the production of heavy quarks (charm and beauty) that are created in the first microseconds after the collisions. The target of this thesis is the measurement of the nuclear modificafion factor and the elliptic flow, described by the v_2 parameter, of the D^+ mesons. I used the data collected in the 2011 Pb-Pb collisions at squared root centre-of-mass energy = 2.76 TeV by the ALICE experiment. The D^+ mesons are detected via the exclusive reconstruction of the three-body decay channel D^+ →K^ − π^ + π ^+. Because of the rarity of the D^+ mesons production is necessary a selection strategy to extract the signal from the large combinatorial background. The exclusive reconstruction of D+ mesons is done by analyzing the invariant mass spectra of decay particles and applying some cuts to distingue the topologies of interest from the background. One of the tasks of this thesis is to tune the topological cuts in order to maximize the statistical significance and to increase the signal-to-background ratio on the real data. The cuts have been tuned in different ranges of the D+ meson transverse momentum. After the tuning of the cut variables the optimal set of cut is applied on the candidates and the raw signal yield was extracted through the invariant mass spectra. The factors of correction that we need to apply to obtain the D+ meson production yield are described and the systematic uncertainties on the D+ meson production yield are also calulated. Finally the results for the nuclear modification factor for central events in the pT range 3-36 GeV/c and for the v_2 in semi-peripheral events in the pT range 3 − 8 GeV/c in Pb-Pb collisions at squared center of mass energy=2.76 TeV are presented.