Measurement of the $\tau$ lepton polarization in the decay $\bar{B}\to D^{\ast }{\tau }^{-}{\bar{\nu }}_{\tau }$

Abstract

We report the first measurement of the $\tau$ lepton polarization in the decay $\bar{B}\to D^{\ast }{\tau }^{-}{\bar{\nu }}_{\tau }$ as well as a new measurement of the ratio of the 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 ${\ell }^{-}$ denotes an electron or a muon, with the decays ${\tau }^{-}\to {\pi }^{-}{\nu }_{\tau }$ and ${\tau }^{-}\to {\rho }^{-}{\nu }_{\tau }$. We use the full data sample of $772\times 10^6$ $B\bar{B}$ pairs accumulated with the Belle detector at the KEKB electron-positron collider. Our preliminary results, $R(D^{\ast })=0.276\pm 0.034{(stat.)}_{-0.026}^{+0.029}(syst.)$ and $P_{\tau }=-0.44\pm 0.47{(stat.)}_{-0.17}^{+0.20}(syst.)$, are consistent with the theoretical predictions of the Standard Model within $0.6$ standard deviation.

31 pages, 6 figures, ICHEP 2016

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• $B$ decay anomalies from nonabelian local horizontal symmetry
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