Measurement of R(D∗) with hadronic τ+ decays at s√ = 13 TeV by the LHCb collaboration

The ratios of branching fractions R(D(∗))=B(B0→D(∗)τντ)/B(B0→D(∗)ℓνℓ), with ℓ=μ,e, allow to test lepton flavour universality in semi-leptonic b-decays and have been measured by BaBar, Belle and LHCb collaborations. The measurements have attracted significant attention as the difference between the experimental world-average values of R(D∗) and R(D) and their theoretical prediction is at the level of three standard deviations. LHCb presented results on R(D) and R(D∗) with muonic τ+ decays in autumn 2022 using the Run 1 data sample. This seminar presents an updated measurement on R(D∗−) using B0→D∗−τ+ντ decays with hadronic τ+ decay channels, based on pp collision data collected by the LHCb collaboration at s√ = 13 TeV in 2015 and 2016, corresponding to an integrated luminosity of 2 fb−1 . This new sample has about 1.5 times more B0 decays than the one used in the previous Run 1 analysis. The excellent vertexing capabilities of the LHCb detector allow for the exploration of interesting features of the three-prong τ+ decays which will be discussed in this talk. This measurement provides one of the most precise single results on R(D∗) to date.