Measurement of the branching ratio of $\bar{B}^0 \rightarrow D^{*+}   \tau^- \bar{\nu}_{\tau}$ relative to $\bar{B}^0 \rightarrow D^{*+} \ell^-   \bar{\nu}_{\ell}$ decays with a semileptonic tagging method
  We report a measurement of ratio ${\cal R}(D^*) = {\cal B}(\bar{B}^0 \rightarrow D^{*+} \tau^- \bar{\nu}_{\tau})/{\cal B}(\bar{B}^0 \rightarrow D^{*+} \ell^- \bar{\nu}_{\ell})$, where $\ell$ denotes an electron or a muon. The results are based on a data sample containing $772\times10^6$ $B\bar{B}$ pairs recorded at the $\Upsilon(4S)$ 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 $D^{*\mp} \ell^{\pm}$. We measure ${\cal R}(D^*)= 0.302 \pm 0.030({\rm stat)} \pm 0.011({\rm syst)}$, which is within $1.6 \sigma$ of the Standard Model theoretical expectation, where $\sigma$ is the standard deviation including systematic uncertainties. 
A. Abdesselam, I. Adachi, K. Adamczyk, H. Aihara, S. Al Said, K.   Arinstein, Y. Arita, D. M. Asner, T. Aso, H. Atmacan, V. ...
hep-ex