Rare dileptonic B0(s) meson decays at LHCb

The B0(s)→ℓℓ¯B(s)0→ℓℓ¯ decays are generated by Flavor Changing Neutral Currents, hence they can proceed only through loop processes. For this reason, and because of an additional helicity suppression, their branching ratios are predicted to be very small in the Standard Model (SM). Nevertheless their rates can be modified by the appearance of New Physics (NP) particles inside the loops. In SM their rates are predicted with a very small uncertainty, and from the comparison between the measured values and their theoretical prediction hints on the NP realization patterns can be inferred. A part of the original work presented in this thesis has been devoted to the optimization of the Multi Variate Analysis (MVA) classifier for the search of the B0(s)→μ+μ−B(s)0→μ+μ− with the full dataset collected at LHCb during the first run of the LHC (corresponding to an integrated luminosity of ∼3fb−1∼3fb−1). This dataset has also been combined with the one collected by the CMS experiment to obtain the first observation of B0s→μ+μ−Bs0→μ+μ−. In view of the update of the analysis aiming to improve the sensitivity for the B0→μ+μ−B0→μ+μ− mode, a new isolation variable, exploiting a topological vertexing algorithm, has been developed and additional studies for a further optimization of the MVA classifier performances have been done. Another original part of the work presented in this thesis concerns the definition of an analysis chain for the search of the B0(s)→τ+τ−B(s)0→τ+τ− modes, which are still largely unexplored. In this thesis the final state where both ττ go into three charged ππ and a ντντ has been studied. The presence of two νν in the final state of the decay makes the reconstruction of the ττ momenta of the two ττ. Nevertheless the possibility of measuring the two decay vertexes of the ττ, as well as the BB candidate production vertex, allows to impose geometrical constraints that can be used in the reconstruction of the ττ momenta. In particular, a new algorithm for the full reconstruction of each event of these momenta and of related variables has been presented and discussed.