Measurement of the Ratio of the B0 → D*− τ+ ν_τ and B0 → D*− μ+ ν_μ Branching Fractions Using Three-Prong τ-Lepton Decays

The ratio of branching fractions R(D*−) ≡ B(B0 → D*− τ+ ν_τ) / B(B0 → D*− μ+ ν_μ) is measured using a data sample of proton-proton collisions collected with the LHCb detector at center-of-mass energies of 7 and 8 TeV, corresponding to an integrated luminosity of 3 fb^−1. For the first time, R(D*−) is determined using the τ-lepton decays with three charged pions in the final state. The B0 → D*− τ+ ν_τ yield is normalized to that of the B0 → D*− π+ π− π+ mode, providing a measurement of B(B0 → D*− τ+ ν_τ) / B(B0 → D*− π+ π− π+) = 1.97 ± 0.13 ± 0.18, where the first uncertainty is statistical and the second systematic. The value of B(B0 → D*− τ+ ν_τ) = (1.42 ± 0.094 ± 0.129 ± 0.054)% is obtained, where the third uncertainty is due to the limited knowledge of the branching fraction of the normalization mode. Using the well-measured branching fraction of the B0 → D*− μ+ ν_μ decay, a value of R(D*−) = 0.291 ± 0.019 ± 0.026 ± 0.013 is established, where the third uncertainty is due to the limited knowledge of the branching fractions of the normalization and B0 → D*− μ+ ν_μ modes. This measurement is in agreement with the standard model prediction and with previous results.