Preprint / hep-ex BELLE-CONF-1608 arXiv:1608.06391

Measurement of the $\tau$ lepton polarization in the decay ${\bar B} \rightarrow D^* \tau^- {\bar \nu_{\tau}}$

Abdesselam, A ; Adachi, I ; Adamczyk, K ; Aihara, H ; Said, S Al ; Arinstein, K ; Arita, Y ; Asner, D M ; Aso, T ; Atmacan, H ; Aulchenko, V ; Aushev, T ; Ayad, R ; Aziz, T ; Babu, V ; Badhrees, I ; Bahinipati, S ; Bakich, A M ; Bala, A ; Ban, Y ; Bansal, V ; Barberio, E ; Barrett, M ; Bartel, W ; Bay, A

2016

Abstract: We report the first measurement of the $\tau$ lepton polarization in the decay ${\bar B} \rightarrow D^* \tau^- {\bar\nu_{\tau}}$ as well as a new measurement of 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, with the decays $\tau^- \rightarrow \pi^- \nu_{\tau}$ and $\tau^- \rightarrow \rho^- \nu_{\tau}$. 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, $R(D^*) = 0.276 \pm 0.034{\rm (stat.)} ^{+0.029} _{-0.026}{\rm (syst.)}$ and $P_{\tau} = -0.44 \pm 0.47 {\rm (stat.)} ^{+0.20} _{-0.17} {\rm (syst.)}$, are consistent with the theoretical predictions of the Standard Model within $0.6$ standard deviation.

Note: Comments: 31 pages, 6 figures, ICHEP 2016

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