Measurement of the $\ddot{I}„$ lepton polarization and $R(D^{\ast })$ in the decay $\bar{B}\to D^{\ast }\ddot{I}{„}^{-}\bar{\hat{I}}{½}_{\ddot{I}}„$

Abstract

We report the first measurement of the $\ddot{I}„$ lepton polarization $P_{\ddot{I}}„(D^{\ast })$ in the decay $\bar{B}\to D^{\ast }\ddot{I}{„}^{-}\bar{\hat{I}}{½}_{\ddot{I}}„$ as well as a new measurement of the ratio of the branching fractions $R(D^{\ast })=\mathcal{B}(\bar{B}\to D^{\ast }\ddot{I}{„}^{-}\bar{\hat{I}}{½}_{\ddot{I}}„)/\mathcal{B}(\bar{B}\to D^{\ast }{\ell }^{-}\bar{\hat{I}}{½}_{\ell })$, where ${\ell }^{-}$ denotes an electron or a muon, and the $\ddot{I}„$ is reconstructed in the modes $\ddot{I}{„}^{-}\to \ddot{I}{€}^{-}\hat{I}{½}_{\ddot{I}}„$ and $\ddot{I}{„}^{-}\to \ddot{I}{}^{-}\hat{I}{½}_{\ddot{I}}„$. We use the full data sample of $772\times 10^6$ $B\bar{B}$ pairs recorded with the Belle detector at the KEKB electron-positron collider. Our results, $P_{\ddot{I}}„(D^{\ast })=-0.38\pm 0.51{(stat.)}_{-0.16}^{+0.21}(syst.)$ and $R(D^{\ast })=0.270\pm 0.035{(stat.)}_{-0.025}^{+0.028}(syst.)$, are consistent with the theoretical predictions of the Standard Model.

7 pages, 2 figures, submitted to Physical Review Letters; one figure was removed from the first version