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Abstract

The branching fraction B(B⁰ →D^*−τ⁺ν_τ) is measured relative to that of the normalization mode B⁰ → D^*−π⁺π⁻π⁺ using hadronic τ⁺ → π⁺π⁻π⁺(π⁰)ν_τ decays in proton-proton collision data at a center-of-mass energy of 13 TeV collected by the LHCb experiment, corresponding to an integrated luminosity of 2  fb⁻¹. The measured ratio is B(B⁰ → D^*−τ⁺ν_τ)/B(B⁰ → D^*−π⁺π⁻π⁺) = 1.70±0.10_−0.10^+0.11, where the first uncertainty is statistical and the second is related to systematic effects. Using established branching fractions for the B⁰ → D^*−π⁺π⁻π⁺ and B⁰ → D^*−μ⁺ν_μ modes, the lepton universality test R(D^*−) ≡ B(B⁰ → D^*−τ⁺ν_τ)/B(B⁰ → D^*−μ⁺ν_μ) is calculated, R(D^*−)=0.247±0.015±0.015±0.012, where the third uncertainty is due to the uncertainties on the external branching fractions. This result is consistent with the Standard Model prediction and with previous measurements.