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 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+ντ ) / 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+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.