Charm production in proton-proton collisions at the LHC with the ALICE detector

The ALICE experiment at CERN will study the medium formed in very high energy lead-lead collisions at the LHC. According to the Quantum Chromo Dynamics theory of the strong interaction, a phase transition to a state where quarks and gluons are not confined into hadrons (Quark-Gluon Plasma) can occur in these collisions. Heavy quarks (charm and beauty) are produced in hard scattering processes in the first stages of the collisions. While travelling through the medium they can lose energy by means of gluon radiation. This affects the momentum spectra of the hadrons produced in the subsequent hadronization. To study the energy loss mechanism and its dependence on the parton nature (quark/gluon) and mass (light/heavy quark), hadron momentum spectra observed in heavy-ion collisions are compared to the same spectra observed in proton-proton collisions, where the formation of a thermalized medium is not expected. In this thesis, the measurement of charm production in proton-proton collisions via the exclusive reconstruction of the D0 meson (cū) decay in the D0->Kπ channel is presented. An invariant mass analysis of selected pairs of particles with opposite charges is performed, exploiting the high spatial resolution of the Inner Tracking System silicon detector, that assures the tracking precision needed to resolve the D0 decay vertices from the interaction point of the colliding protons (primary vertex). The estimate of the fraction of secondary D0 mesons coming from the decay of B mesons, if based on theoretical calculations, introduces a relevant systematic error. A method to extract this contribution directly from data, via the analysis of the impact parameter distribution, i.e. the distance between the reconstructed meson trajectory and the primary vertex, is presented. The presence of the ITS assures the possibility to highlight the different shape of the impact parameter distribution typical of secondary D0 mesons, influenced by the relative long lifetime of B mesons (cτ~460-490 μm). Due to construction and mounting finite precision, the modules composing the ITS can be displaced from their designed positions by large amounts if compared with the expected spatial resolutions, of the order of tens of micron. This deteriorates the detector performances, in particular the tracking spatial and momentum resolutions. The alignment of the ITS, that is, the a posteriori determination of more than 13000 parameters defining the real position and orientation in space of the modules, is extensively discussed. The results obtained with 2008 cosmic-ray data are presented. The promising alignment status allowed to test the analysis for the D0 reconstruction on proton-proton collision data simulated with realistic detector properties.