Test of Lepton Flavor Universality by the Measurement of the B0 → D∗− τ+ ν_τ Branching Fraction Using Three-Prong τ 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. The τ lepton is reconstructed with three charged pions in the final state. A novel method is used that exploits the different vertex topologies of signal and backgrounds to isolate samples of semitauonic decays of b hadrons with high purity. Using the B0 → D*− π+ π− π+ decay as the normalization channel, the ratio B(B0 → D*− τ+ ν_τ) / B( B0 → D*− π+ π− π+) is measured to be 1.97 ± 0.13 ± 0.18, where the first uncertainty is statistical and the second systematic. An average of branching fraction measurements for the normalization channel is used to derive B(B0 → D*− τ+ ν_τ) = (1.42 ± 0.094 ± 0.129 ± 0.054)%, where the third uncertainty is due to the limited knowledge of B(B0 → D*− π+ π− π+). A test of lepton flavor universality is performed using the well-measured branching fraction B(B0 → D*− μ+ ν_μ) to compute R(D*−) = 0.291 ± 0.019 ± 0.026 ± 0.013, where the third uncertainty originates from the uncertainties on B(B0 → D*− π+ π− π+) and B(B0 → D*− μ+ ν_μ). This measurement is in agreement with the Standard Model prediction and with previous measurements.