Measurement of the branching ratio of ${\bar{B}}^0\to D^{\ast +}{\tau }^{-}{\bar{\nu }}_{\tau }$ relative to ${\bar{B}}^0\to D^{\ast +}{\ell }^{-}{\bar{\nu }}_{\ell }$ decays with a semileptonic tagging method

Abstract

We report a measurement of ratio $\mathcal{R}(D^{\ast })=\mathcal{B}({\bar{B}}^0\to D^{\ast +}{\tau }^{-}{\bar{\nu }}_{\tau })/\mathcal{B}({\bar{B}}^0\to D^{\ast +}{\ell }^{-}{\bar{\nu }}_{\ell })$, where $\ell$ denotes an electron or a muon. The results are based on a data sample containing $772\times 10^6$ $B\bar{B}$ pairs recorded at the ${\rm Y}(4S)$ resonance with the Belle detector at the KEKB $e^{+}{e}^{-}$ collider. We select a sample of $B^0{\bar{B}}^0$ pairs by reconstructing both $B$ mesons in semileptonic decays to $D^{\ast \mp }{\ell }^{\pm }$. We measure $\mathcal{R}(D^{\ast })=0.302\pm 0.030(\mathrm{stat})\pm 0.011(\mathrm{syst})$, which is within $1.6\sigma$ of the Standard Model theoretical expectation, where $\sigma$ is the standard deviation including systematic uncertainties.

arXiv comment12pages, 13 figures, Moriond EW 2016

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