Measurement of the Ratio of Branching Fractions B(B[over ¯]^0→D^*+τ^-ν[over ¯]_τ)/B(B[over ¯]^0→D^*+μ^-ν[over ¯]_μ)

The branching fraction ratio R(D^*)≡B(B[over ¯]^0→D^*+τ^-ν[over ¯]_τ)/B(B[over ¯]^0→D^*+μ^-ν[over ¯]_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[over ¯]_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[over ¯]^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.