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

The ratios of branching fractions R(D*) ≡ B(anti-B → D* τ− anti-ν_τ) / B(anti-B → D* μ− anti-ν_μ) and R(D0) ≡ B(B− → D0 τ− anti-ν_τ) / B(B− → D0 μ− anti-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− ν_τ anti-ν_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.