Thématique : Physique des particules
The study of these modes is motivated by several anomalies which have been revealed somewhat recently in some experimental measurements, but also by phenomenological analyses. In fact, exciting, persistent hints of New Physics (NP) have shown up in analyses of rare B decays proceeding via flavor-changing neutral currents performed by LHCb and other experiments. All these measurements have received considerable attention as they provide deviations building up a tension at the 4-5σ level with respect to the Standard Model (SM) and are currently the only indications of new physics coming from the CERN Large Hadron Collider (LHC) [1,2]. If none of these results is yet an evidence of NP, the picture they convey is impressively coherent. Their interpretations suggest consistently that: the heavy τ lepton should be more affected by NP than the lighter e and μ leptons; the b → s ττ transition could be enhanced up to 3 orders of magnitude with respect to the SM predictions; NP appears not to respect the SM paradigm of Lepton Flavor Universality, leading in many models to a Violation of Lepton Flavor as well, with measurable rates for and b → s τμ transitions.
Lepton Flavor (Univerality) Violation is hence becoming an extremely hot topic on which LHCb should be able to shed light in the forthcoming years. The very rare decays of B(s) mesons into tau leptons are key channels to indirectly constraint the new physics phase space, though extremely difficult to study.
The LHCb experiment, one of the four large experiments operating at the LHC, is dedicated to heavy flavor physics. During the LHC Run I and II, it has collected about 9 fb−1 of proton-proton interactions, gathering the largest sample of B mesons ever recorded, and thus it is a unique place to look for rare or forbidden decays.
After taking a leading role in the first observation of the B0s → μ+ μ− decay, after a 20 years search (published in Nature in 2015 ), the LHCb team has published the first limit on B0s → τ+ τ− in 2017  and has presented a soon to be published first result ever for the B0s → μτ mode  with the Run I data. This last results is on the verge of excluding (or otherwise confirm) some of the most promising models with Leptoquarks explaining the above-mentioned anomalies. The goal of this thesis is to improve and optimize the analysis technique and strategy for these modes while exploiting the full LHCb data-set as well as the development of new reconstruction algorithms.
(2) Eur. Phys. J. C (2017) 77: 377