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

Abstract

The ratio of branching fractions R(D*⁻)≡B(B⁰→D*⁻τ⁺ν[subscript τ])/B(B⁰→D*⁻μ⁺ν[subscript 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 B^{0}→D*⁻τ⁺ν[subscript τ] yield is normalized to that of the B⁰→D*⁻π⁺π⁻π⁺ mode, providing a measurement of B(B⁰→D*⁻τ⁺ν[subscript 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[subscript 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[subscript 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[subscripts μ] modes. This measurement is in agreement with the standard model prediction and with previous results.