With the full data sample of $772 \times 10^6$ $B{ \bar B}$ pairs recorded by the Belle detector at the KEKB electron-positron collider, the decay $\bar{ B} \rightarrow D^* \tau^- \bar{ \nu}_\tau$ is studied with the hadronic $\tau$ decays $\tau^- \rightarrow \pi^- \nu_\tau$ and $\tau^- \rightarrow \rho^- \nu_\tau$. The $\tau$ polarization $P_\tau(D^*)$ in two-body hadronic $\tau$ decays is measured, as well as the ratio of the branching fractions $R(D^{ *}) = \mathcal{ B}(\bar { B} \rightarrow D^* \tau^- \bar{ \nu}_\tau) / \mathcal{ B}(\bar{ B} \rightarrow D^* \ell^- \bar{ \nu}_\ell)$, where $\ell^-$ denotes an electron or a muon. Our results, $P_\tau(D^*) = -0.38 \pm 0.51 { \rm (stat)} ^{ +0.21}_{ -0.16} { \rm (syst)}$ and $R(D^*) = 0.270 \pm 0.035{ \rm (stat)} ^{ +0.028}_{ -0.025}{ \rm (syst)}$, are consistent with the theoretical predictions of the Standard Model. The polarization values of $P_\tau(D^*) > +0.5$ are excluded at the 90\% confidence level.
Categories: hep-ex