B -> D(*) tau nu overview
A measurement of the ratio of semileptonic branching fractions R(D∗−)≡B(B0→D∗−τν)/B(B0→D∗−μν)
is performed using a data sample of proton-proton collisions, corresponding to an integrated luminosity of 3 fb−1, collected
by the LHCb experiment at center-of-mass energies of 7 and 8 TeV.For the first time, R(D∗−)is determined using the τ lepton decay with three charged pions in the final state. The B0→D∗τν yield is normalized to that of the B0→D∗−πππ mode. A measurement of
B(B0→D∗−τν)/B(B0→D∗−πππ) =1.93±0.13±0.17 is obtained, where the first uncertainty is statistical and the second systematic.
The value of B(B0→D∗−τν) = (1.39±0.09±0.12±0.06)% is obtained, where the third uncertainty is due to the limited knowledge of the branching fraction of the normalization mode. Using the well-measured branching fraction of the B0→D∗−μν decay, a value of R(D∗−) =
0.285±0.019±0.025±0.013 is established, where the first uncertainty is statistical, the second systematic, and the third is due to the limited knowledge of the branching fractions of the normalization and of the B0→D∗−μν modes. This measurement is in agreement with
the Standard Model prediction and with previous results.