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

TL;DR

This paper presents the first measurement of the tau lepton polarization and the ratio of branching fractions in B meson decays, using Belle detector data, with results consistent with Standard Model predictions.

Contribution

It provides the first measurement of tau polarization in B decays and refines the ratio of branching fractions using a large data sample.

Findings

Measured R(D*) = 0.276 ± 0.034(stat) +0.029/-0.026(syst)

Measured tau polarization P_tau = -0.44 ± 0.47(stat) +0.20/-0.17(syst)

Results are consistent with Standard Model within 0.6 standard deviations.

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.