Test of lepton flavor universality using B0→D*−τ+ντ decays with hadronic τ channels

The branching fraction B(B0→D∗−τ+ντ) is measured relative to that of the normalisation mode B0→D∗−π+π−π+ using hadronic τ+→π+π−π+(π0)ν¯τ decays in proton-proton collision data at a centre-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+ντ)/B(B0→D∗−π+π−π+)=1.70±0.10+0.11−0.10, 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+νμ modes, the lepton universality test, R(D∗−)≡B(B0→D∗−τ+ντ)/B(B0→D∗−μ+νμ) 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.