Recherche des désintégrations violant la saveur leptonique Bs -> tµ et Bd -> tµ avec l'expérience LHCb

Abstract : The decay $B_{ (s)}$→τμ is suppressed in the SM, in which lepton flavour is conserved. Its observation would therefore be an unambiguous evidence of physics beyond the SM. Recent results [1,2] revived the interest for the search of such processes [3]. This thesis presents the search for the $B_{ (s)}$→τμ decays within the LHCb experiment, one of the 4 large experiments operated at the CERN Large Hadron Collider (LHC).The τ lepton decays before reaching the LHCb detector and is reconstructed using the τ→πππν channel. The neutrino from the τ decay escapes detection. A specific reconstruction technique is used in order to infer the energy of the ν and thus the invariant mass of the decaying B meson. This way, the complete kinematics of the process can be solved up to a two fold ambiguity.In order to disentangle signal from background, an offline selection consisting of different steps is applied. Data driven and multivariate analysis techniques, such as Boosted Decision Trees (BDT), are used during the selection process. The analysis strategy is completed by a simultaneous fit to the B meson invariant mass distribution over the different bins of a final BDT. According to the SM expectations, no signal events should be observed. In this case, the CLs method will be used to extract the upper limits on the branching fractions.[1] Test of lepton universality using $B^+$→$K^+$$l ^+$$l ^-$ decaysPhys. Rev. Lett. 113, 151601 (2014)[2] Measurement of the ratio of branching fractions BR(B → D*τ ν)/BR(B → D*μν)Phys. Rev. Lett. 115 (2015) 111803[3] Lepton Flavour Violation in B Decays ? Phys. Rev. Lett. 114, 091801 (2015)
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Joan Arnau Romeu. Recherche des désintégrations violant la saveur leptonique Bs -> tµ et Bd -> tµ avec l'expérience LHCb. Physique des Hautes Energies - Expérience [hep-ex]. Aix Marseille Université, 2018. Français. ⟨tel-02265292⟩

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