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

The ratios of branching fractions R(D∗)≡B(¯B→D∗τ−¯ντ)/B(¯B→D∗μ−¯νμ) and R(D0)≡B(B−→D0τ−¯ντ)/B(B−→D0μ−¯νμ) 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−ντ¯νμ. 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.