Measurement of the $τ$ lepton polarization in the decay B → D^* τ⁻ ν_τ

We report the first measurement of the $τ$ lepton polarization 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, with the decays τ⁻ → π⁻ ν_τ and τ⁻ → ρ⁻ ν_τ. We use the full data sample of 772 × 10⁶ B B pairs accumulated with the Belle detector at the KEKB electron-positron collider. Our preliminary results, R(D^*) = 0.276 ± 0.034 (stat.) +0.029 -0.026 (syst.) and P_τ = -0.44 ± 0.47 (stat.) +0.20 -0.17 (syst.), are consistent with the theoretical predictions of the Standard Model within $0.6$ standard deviation.