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

Abstract

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