Beauty in the Crowd: Commissioning of the LHCb Pile-Up detector and First evidence of $B^0_s\rightarrow\mu^+\mu^-$

Our current understanding of matter at a fundamental level is encompassed in the Standard Model (SM) of particle physics. Despite proved to be very successful in describing existing data and predicting the outcome of experiments, the model still leaves open questions. The missing answers might lie beyond the Standard Model, in models that we call “New Physics” (NP). The mission of particle physics, hoc die, is to search for phenomena that cannot be explained by the SM and one way to proceed is to look for indirect manifestations of NP. This is the approach chosen by the LHCb experiment, a “beauty” Large Hadron Collider experiment focusing in particular on b and c-meson decays. LHCb is a single-arm forward spectrometer, consisting of several subdetectors designed to trigger and reconstruct heavy flavour decay events. Among the subdetectors of the LHCb experiment, the Pile-Up (PU) is designed to detect multiple interactions in proton beam collisions and to reject these events already at the trigger level, as they are expected to create a difficult environment for b-physics analyses. Although the use of the Pile-Up detector was changed from its original intention, the system has been operational since the beginning of the experiment data acquisition and was included in the trigger selection. The detector was used mainly to reconstruct beam-gas interactions and to measure the instantaneous luminosity of the colliding beams, as illustrated in this dissertation. In December 2012 LHCb completed the first data taking run. Due to its vertexing precision, mass resolution and particle identification capabilities, a precise reconstruction and background-free selection of many B-decays topologies were realised. Programs for searches of New Physics manifestations were carried out. A promising field to look for New Physics is to investigate processes that are forbidden or strongly suppressed in the Standard Model, due to the fact that they can only occur at loop level. Where the SM contribution is tiny, New Physics contributions can be relatively large. In this case, any deviation from the SM prediction can be directly connected to NP. In particular, such loop processes might be affected by particles with a mass significantly higher than the energy available through the LHC beams. In this framework, the decay of a neutral B meson in two muons is often considered the golden channel in probing New Physics via quantum fluctuations. The $B^0_{s(d)}\rightarrow\mu^+\mu^-$ branching fraction, predicted to be about $3\times10^{−9}$ in the Standard Model, might in fact be enhanced of up to even one order of magnitude in some New Physics models. This, and the cleanliness of the theoretical calculations, justifies the large interest shown by both experimental and theoretical communities towards this particular rare decay. The quest for $B^0_d\rightarrow\mu^+\mu^-$ decays started at the Belle and BaBar experiments, that of $B^0_s\rightarrow\mu^+\mu^-$ at CDF and D0, but both have been unsuccessful so far. The first evidence for the $B^0_s\rightarrow\mu^+\mu^-$ decay presented in this dissertation came only recently from the LHCb collaboration and is one of the main particle physics achievements of 2012. The measurement of its branching fraction leads to another apparently successful test of the SM, but the possible contribution of New Physics to the decay cannot be excluded yet. Only a higher statistics will allow to improve the experimental precision of the result and to explore new observables describing the decay.