New physics from rare beauty

The Standard Model (SM) of particle physics offers a framework to describe sub-nuclear physics processes. Despite its success in describing a large variety of sub-nuclear phenomena, the SM leaves open a number of questions and is regarded by particle physicists as an effective quantum field theory, rather than a credible candidate to be the ultimate theory of fundamental interactions. Particle physicists postulates the existence of New Physics (NP) beyond the SM. This would result into the creation of new particles that could then be observed at higher energy regimes than hitherto explored. These particles could be observed directly or indirectly, through their contributions to quantum loops. A promising ground to look for NP are the Flavor Changing Neutral Currents (FCNCs). Due to its precise theoretical prediction, one of the most promising rare decay channel is the $B_s$ meson decaying into two muons. In this dissertation the strategy for an early measurement of the $B_{s} \rightarrow \mu \mu$ branching ratio at the "Large Hadron Collider beauty" (LHCb) experiment is illustrated. LHCb is one the four particle physics detector experiments being constructed at the Large Hadron Collider (LHC) accelerator at CERN (European Organization for Nuclear Research). It is the only LHC experiment explicitly designed to study CP violation and rare B hadron decays. The potential of LHCb at excluding the SM $B_{s} \rightarrow \mu \mu$ branching ratio at 90% CL with 2~$\mathrm{fb^{-1}}$ of integrated luminosity resulted: $Br^{90%CL}(B_{s} \rightarrow \mu \mu) = (1.86^{+1.20}_{-0.84}) \times 10^{-9}$. Systematic checks showed that the developed method was robust against residual misalignments. The luminosity needed to reach a 3$\sigma$ evidence (5$\sigma$ discovery) with respect to the SM prediction was estimated to be 4~$\mathrm{fb^{-1}}$ (resp. 10~$\mathrm{fb^{-1}}$)%. If the $Br(Bs \to\mu \mu)$ is enhanced by new-physics, the measurement of the branching ratio could be achieved sooner than expected. For instance, assuming the most probable scenario predicted by the NUHM model - $Br(B_{s} \rightarrow \mu \mu) \sim 10^{-8}$ - the $5\sigma$ discovery could be achieved in the first year of LHCb data, with about 2~$\mathrm{fb^{-1}}$ of integrated luminosity. Moreover, the LHCb Outer Tracker (OT) sub-detector, covering most of the tracking volume, is described in detail including an overview of the OT ageing phenomena and the Quality Assurance of the Front-End Electronics production. Detailed studies of the systematic uncertainties related to the OT geometry and B-field transformation (weak modes), leading to a deterioration of the track reconstruction performance are described.