Measurement of the Ratios of Branching Fractions R (D∗) and R (D0)

The ratios of branching fractions R(D∗)B(B¯→D∗τ-ν¯τ)/B(B¯→D∗μ-ν¯μ) and R(D0)B(B-→D0τ-ν¯τ)/B(B-→D0μ-ν¯μ) are measured, assuming isospin symmetry, 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-ντν¯μ. The measured values are R(D∗)=0.281±0.018±0.024 and R(D0)=0.441±0.060±0.066, where the first uncertainty is statistical and the second is systematic. The correlation between these measurements is ρ=-0.43. The results are consistent with the current average of these quantities and are at a combined 1.9 standard deviations from the predictions based on lepton flavor universality in the standard model.