Measurement of the τ lepton polarization and R(D*) in the decay B¯→D*τ−ν¯τ with one-prong hadronic τ decays at Belle

With the full data sample of 772×106 BB̄ pairs recorded by the Belle detector at the KEKB electron-positron collider, the decay B̄→D∗τ-ν̄τ is studied with the hadronic τ decays τ-→π-ντ and τ-→ρ-ντ. The τ polarization Pτ(D∗) in two-body hadronic τ decays is measured, as well as the ratio of the branching fractions R(D∗)=B(B̄→D∗τ-ν̄τ)/B(B̄→D∗-ν̄), where - denotes an electron or a muon. 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. The polarization values of Pτ(D∗)>+0.5 are excluded at the 90% confidence level.