Rare dileptonic B0(s) meson decays at LHCb

Abstract : The $B^0_{(s)}\rightarrow \ell\bar{\ell}$ 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 $B^0_{(s)}\rightarrow\mu^+\mu^-$ with the full dataset collected at LHCb during the first run of the LHC (corresponding to an integrated luminosity of $\sim 3fb^{-1}$). This dataset has also been combined with the one collected by the CMS experiment to obtain the first observation of $B^0_s\rightarrow\mu^+\mu^-$. In view of the update of the analysis aiming to improve the sensitivity for the $B^0\rightarrow\mu^+\mu^-$ 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 $B^0_{(s)}\rightarrow\tau^+\tau^-$ modes, which are still largely unexplored. In this thesis the final state where both $\tau$ go into three charged $\pi$ and a $\nu_\tau$ has been studied. The presence of two $\nu$ in the final state of the decay makes the reconstruction of the $\tau$ momenta of the two $\tau$. Nevertheless the possibility of measuring the two decay vertexes of the $\tau$, as well as the $B$ candidate production vertex, allows to impose geometrical constraints that can be used in the reconstruction of the $\tau$ 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.
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Alessandro Mordà. Rare dileptonic B0(s) meson decays at LHCb. High Energy Physics - Experiment [hep-ex]. Aix-Marseille Université, 2015. English. ⟨tel-01252492v2⟩

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