Test of lepton flavor universality by the measurement of the B0→D*−τ+ντ branching fraction using three-prong τ decays

The ratio of branching fractions (D∗−)≡(B0→D∗−τ+ντ)/(B0→D∗−μ+νμ) 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 (B0→D∗−τ+ντ)/(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 (B0→D∗−τ+ντ)=(1.42±0.094±0.129±0.054)%, where the third uncertainty is due to the limited knowledge of (B0→D∗−π+π−π+). A test of lepton flavor universality is performed using the well-measured branching fraction (B0→D∗−μ+νμ) to compute (D∗−)=0.291±0.019±0.026±0.013, where the third uncertainty originates from the uncertainties on (B0→D∗−π+π−π+) and (B0→D∗−μ+νμ). This measurement is in agreement with the Standard Model prediction and with previous measurements.