Test of lepton flavor universality using B0 --> D*- tau+ nutau decays with hadronic tau channels

Abstract

The branching fraction B(B0 --> D*- tau+ nutau) is measured relative to that of the normalization mode B0 --> D*- pi+ pi- pi+ using hadronic tau+ --> pi+ pi- pi+ (pi0) antinutau 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*- tau+ nutau)/ B(B0 --> D*- pi+ pi- pi+) = 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*- pi+ pi- pi+ and B0 --> D*-mu+ numu modes, the lepton universality test R(D*-) = B(B0 --> D*- tau+ nutau) / B(B0 --> D*- mu+ numu) 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.