Recent $\bar{B}\to D^{(\ast )}{\tau }^{-}{\bar{\nu }}_{\tau }$ Studies at Belle

Abstract

The semitauonic decay $\bar{B}\to D^{\ast }{\tau }^{-}{\bar{\nu }}_{\tau }$ is sensitive to new physics beyond the Standard Model (SM) that has an enhanced coupling to the $\tau$ lepton. In the ratio of branching fractions $R(D^{\ast })=\mathcal{B}(\bar{B}\to D^{\ast }{\tau }^{-}{\bar{\nu }}_{\tau })/\mathcal{B}(\bar{B}\to D^{\ast }{\ell }^{-}{\bar{\nu }}_{\ell })$, where ${\ell }^{-}=e^{-}$ or ${\mu }^{-}$, a 3.3$\sigma$ anomaly was observed. In order to investigate the anomaly further, Belle performed a new $R(D^{\ast })$ measurement using one-prong hadronic $\tau$ decays, which was statistically independent of the previous two measurements. This measurement included the first measurement of the $\tau$ polarization $P_{\tau }(D^{\ast })$ using the kinematics of the two-body decays. The obtained results, $R(D^{\ast })=0.270\pm 0.035(\mathrm{stat}{)}_{-0.025}^{+0.028}(\mathrm{syst})$ and $P_{\tau }(D^{\ast })=-0.38\pm 0.51(stat{)}_{-0.16}^{+0.21}(syst)$, were consistent both with the SM and the world-average $R(D^{\ast })$. Including this result, the $R(D^{\ast })$ anomaly became 3.4$\sigma$ away from the SM prediction.

4 pages, 2 figures, proceedings of 29th Rencontres de Blois, 28 May - 2 June 2017