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Abstract

The measurement of the differential branching fraction and an angular analysis of the decay B s 0 → φμ + μ − is performed, using a dataset collected by the LHCb experiment in proton-proton collisions in 2011 and 2012, corresponding to an integrated luminosity of 3 fb −1 . The results are presented as function of q 2 , the square of the dimuon invariant mass. A full set of angular observables is extracted for the first time for this decay mode, where all measured angular coefficients are consistent with Standard Model expectations. For the range of 1 < q 2 < 6 GeV 2 /c 4 the branching fraction is found to be more than 3 standard deviations below the Standard Model prediction, which is derived using a combination of light-cone sum rules and lattice QCD calculations. This tension is compatible with observations made by other measurements of electroweak penguin decays mediated by b → s + − transitions in this q 2 region. Possible explanations are theoretical models which predict new heavy degrees of freedom at the TeV scale, as well as underestimated contributions from cc-loop corrections in the SM calculations. An important aspect of the measurements of branching fractions is the determination of the track reconstruction efficiency. It is determined from J/ψ → μ + μ − decays, where the data-driven tag-and-probe technique is used. During LHC Run II this measurement is performed in a “quasi-online” approach, directly after the data has been recorded. For the 2015 data-taking period of the LHCb experiment, the track reconstruction efficiency is found to be (96.14 ± 0.03)%, where the uncertainty is statistical only.