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 $b\rightarrow c (W^{-} \rightarrow \overline{c}s)$ transition. This thesis presents the study of $B^{0}\rightarrow D^{0}\overline{D}^{0}K^{\ast 0}$ decay which has never been observed so far. The branching ratio is quoted with respect to the $B^{0}\rightarrow D^{\ast -} D^{0} K^{+}$ decay mode. No $K^{\ast0}$ mass window selection is applied in sigmode, reconstructing the $K\pi$ system as a $K^{\ast0}$. The invariant mass of the $K\pi$ system is selected to be in full allowed phase space: $ m(K) + m(\pi) < m(K\pi) < m(B)-2 m(D^{0})$. $D^{0}$ mesons are reconstructed through the Cabibbo favoured $D^{0}\rightarrow K^{-}\pi^{+}$ mode and the $K^{\ast 0}$ as $K^{+}\pi^{-}$. The integrated luminosity of 3fb$^{-1}$ collected by LHCb during LHC Run 1 are used to select and reconstruct $B{0}\rightarrow D^{0}\overline{D}^{0} K^{\ast 0}$ leading to a preliminary branching ratio corresponding to: $$\dfrac{\mathcal{B}(B{0}\rightarrow D^{0}\overline{D}^{0} K^{\ast 0})}{\mathcal{B}(B^{0}\rightarrow D^{\ast -} D^{0} K^{+})} = ( 12.83 \pm 1.80\mbox{(stat)} ) \%$$ LHCb foreseen a major upgrade for the 2020. At that time, LHCb will operate at five times larger luminosity than Run I reaching the value of $\mathcal{L}=2\times 10^{33}$ lumiunits. 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 5fb$^{-1}$ 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 \KS and $\Lambda^{0}$. 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.