Abstract

We report a measurement of the 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 the standard deviation $\sigma$ includes systematic uncertainties. We use this measurement to constrain several scenarios of new physics in a model-independent approach.