Measurement of the τ Lepton Polarization and R (D∗) in the Decay B →d∗τ- ν τ

We report the first measurement of the τ lepton polarization Pτ(D?) in the decay B→D?τ-ντ as well as a new measurement of the ratio of the branching fractions R(D?)=B(B→D?τ-ντ)/B(B→D?-ν), where - denotes an electron or a muon, and the τ is reconstructed in the modes τ-→π-ντ and τ-→ρ-ντ. We use the full data sample of 772×106 BB pairs recorded with the Belle detector at the KEKB electron-positron collider. Our results, Pτ(D?)=-0.38±0.51(stat)-0.16+0.21(syst) and R(D?)=0.270±0.035(stat)-0.025+0.028(syst), are consistent with the theoretical predictions of the standard model. ? 2017 American Physical Society.