We report the first measurement of the τ lepton polarization Pτ (D ∗ ) in the decay B¯ → D ∗ τ −ν¯τ as well as a new measurement of the ratio of the branching fractions R(D ∗ ) = B(B¯ → D ∗ τ −ν¯τ )/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.) +0.21 −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.
Physical Review Letters
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