With the full data sample of 772×106 B¯B pairs recorded by the Belle detector at the KEKB electron-positron collider, the decay ¯B→D∗τ−¯ντ 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(¯B→D∗τ−¯ντ)/B(¯B→D∗ℓ−¯νℓ), where ℓ− denotes an electron or a muon. Our results, Pτ(D∗)=−0.38±0.51(stat)+0.21−0.16(syst) and R(D∗)=0.270±0.035(stat)+0.028−0.025(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.
Physical Review D
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