Study of double charm B decays with the LHCb experiment at CERN and track reconstruction for the LHCb upgrade

Double charmed B meson decays are dominated by the Cabibbo favoured b → c(W^- → c-bar s) transition. This thesis presents the study of B⁰ → D⁰D-bar⁰K*⁰ decay which has never been observed so far. The branching ratio is quoted with respect to the B⁰ → D*^-D⁰K⁺ decay mode. No K*⁰ mass window selection is applied in B⁰ → D⁰D-bar⁰K*⁰, reconstructing the Kπ system as a K*⁰. The invariant mass of the Kπ system is selected to be in full allowed phase space: m(K) + m(π) < m(Kπ) < m(B) - 2m(D⁰). D⁰ mesons are reconstructed through the Cabibbo favoured D⁰ → K^-π⁺ mode and the K*⁰ as K⁺π^-. The integrated luminosity of 3 fb^-1 collected by LHCb during LHC Run 1 are used to select and reconstruct B⁰ → D⁰D-bar⁰K*⁰ leading to a preliminary branching ratio corresponding to: B(B⁰ → D⁰D-bar⁰K*⁰)/B(B⁰ → D*^-D⁰K⁺) = (12.83 ± 1.80(stat))%. LHCb foresees a major upgrade for the 2020. At that time, LHCb will operate at five times larger luminosity than Run I reaching the value of L = 2 x 10³³. An increased pile-up level is expected leading to higher detector occupancy as well as harsher radiation environment. A new trigger strategy will be adopted to take advantage of the higher luminosity to collect at least 5 fb^-1 per year. The hardware based trigger strategy used during Run I and Run II will be completely removed and a fully based software trigger strategy will be adopted. Therefore, software applications performing trigger selection will be executed at collision rate. Such strategy requires the replacement of all the read-out electronics in all the subsystem and in order to guarantee high track reconstruction performance all the tracking sub-detectors will be replaced. The tracker placed downstream the LHCb dipole magnet will be replaced by a scintillating fibre tracker (SciFi) made of layers of scintillating fibres read-out by Silicon Photo Multiplier. This thesis presents the LHCb upgrade, the LHCb upgrade tracking strategy as well as the development of the stand-alone track reconstruction algorithm using only information from the SciFi. This algorithm plays a crucial role in the reconstruction of particles originating from decaying b and c hadrons as well as particles originating from long-lived particles such as K⁰_S and Λ⁰. The algorithm strongly enhances the overall expected performance for the upgrade leading to a large improvement in reconstruction efficiency, fake tracks rejection and execution time. (author)