ricostruzione dei mesoni D+ tramite il decadimento in k-pi+pi+ in collisioni pp a sqrt{s} = 7 TeV

The target of this thesis is the measurement of the cross-section for the production of D+ mesons with the first data collected by the ALICE experiment in proton-proton collisions at sqrt{s} = 7 TeV via the exclusive reconstruction of the decay channel D+ &#8722;&#8594; K &#8722; &#960; + &#960; + .A general introduction on the heavy-ion physics is given in Chapter 1. The physical observables useful for such kinds of analyses and the most significant results from RHIC and ALICE are presented. Chapter 2 is devoted to open charm physics and to the production of open charm in heavy-ion and p-p collisions. An overview of the most recent results on open charm measurements is also given. A brief description of the ALICE detectors, software and performance of tracking and vertexing are presented in Chapters 3 and 4. In the last two Chapters the principal analyses developed in this thesis work on the exlusive reconstruction of D+ mesons are presented.The exclusive reconstruction of D+ mesons is done by analyzing the invariant mass spectrum of decay particles. Detected particles are combined into candidate topologies that correspond to the studied decay channel. Due to the very small signal-to-background ratio, this simple technique is not applicable in the case of rare particles such as charmed mesons. The only solution is to find some properties of the real topologies that allow distinguishing the signal from the combinatorial background and applying cuts based on them. The main variables used to separate the signal from combinatorial background of the three charged track combinations are the distance between the primary and secondary vertex, the cosine of pointing angle (&#952;point ) and the the sum of the three impact paramenters. Thus, one of the main 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 sets of cut are applied on the candidate and the raw signal yield was extracted through the invariant mass spectra. Different methods of extraction were developed and compared. The statistical error affecting the recovered yield of produced D+ decayed in the D+ &#8722;&#8594; K &#8722; &#960; + &#960; + channel and the invariant mass resolution are estimated. The measured yield is then corrected for detector acceptance, cut selection efficiency and for feed down from B-mesons extracted from MC simulation with detailed description of detector response and experimental conditions. As a further studies a detailed analyses on the systematic error introduced by the raw yield extraction and by the correction efficiencies are described. Finally the preliminary results on the measurement of the production cross-section of the D+ mesons at central rapidity in p-p collisions at 7 TeV in 2 < pT < 12 GeV/c is reported. pQCD predictions (FONLL and GM-VFNS) are compatible with our data. These measurements will provide reference data to measure the energy loss of D-mesons in Pb-Pb collisions at the LHC.