Measurement of the ratio of branching fractions B(B¯⁰ → D^∗+τ^-ν¯_τ) / B(B¯⁰ → D^∗+μ^-ν¯_μ)

The branching fraction ratio R(D^∗)≡B(B¯⁰→D^∗+τ^-ν¯_τ) / B(B¯⁰→D^∗+μ^-ν¯_μ) is measured using a sample of proton-proton collision data corresponding to 3.0 fb^-1 of integrated luminosity recorded by the LHCb experiment during 2011 and 2012. The tau lepton is identified in the decay mode τ^-→μ^-ν¯_μν_τ. The semitauonic decay is sensitive to contributions from non-standard-model particles that preferentially couple to the third generation of fermions, in particular, Higgs-like charged scalars. A multidimensional fit to kinematic distributions of the candidate B¯⁰ decays gives R(D^∗)=0.336±0.027(stat)±0.030(syst). This result, which is the first measurement of this quantity at a hadron collider, is 2.1 standard deviations larger than the value expected from lepton universality in the standard model.