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Title: Search for the rare fully leptonic decay B+ → μ+ μ− μ+ ν at LHCb
Author: Stefkova, Slavomira
ISNI:       0000 0004 7658 6511
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2018
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This thesis presents a first search for the fully leptonic decay B+ → μ+ μ− μ+ ν in any experiment. This search is performed using proton-proton collision data at LHCb corresponding to an integrated luminosity of 4.7 fb^{−1} . The search is carried out in the region where the minimum of the two μ+ μ− mass combinations is below 980 MeV/c^{2}. The measurement of the branching fraction of this decay is even more interesting given that the recent theoretical prediction [1] of the branching fraction for B+ → μ+ μ− μ+ ν of 1.3 × 10^{−7} is high. Moreover, this decay is sensitive to the magnitude of the coupling strength between b and u quarks, which is of great interest given that there are some tensions in measurements of this magnitude. The data are consistent with the background only hypothesis and a limit of 1.4×10^{−8} at 95% confidence level is set on the branching fraction in the stated kinematic region. This is therefore not consistent with the theoretical prediction made in Ref. [1]. This thesis also presents a study of the response of the detector if three muons pass through it. This study shows that correlations induced by a trimuon system in the detector are substantial and they need to be addressed properly.
Supervisor: Egede, Ulrik Sponsor: Science and Technology Facilities Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral