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

Here, we report a measurement of the ratio $R\left(D^{*}\right)=B({\overline{B}}^0\to D^{*+}{\tau }^{-}{\overline{\nu }}_{\tau })/B({\overline{B}}^0\to D^{*+}{\ell }^{-}{\overline{\nu }}_{\ell })$, where $\ell$ denotes an electron or a muon. The results are based on a data sample containing $772\times 10^{6}B\overline{B}$ pairs recorded at the ${\rm Y}\left(4S\right)$ resonance with the Belle detector at the KEKB $e^{+}{e}^{-}$ collider. We select a sample of $B^0{\overline{B}}^0$ pairs by reconstructing both $B$ mesons in semileptonic decays to $D^{*\mp }{\ell }^{\pm }$. We measure $R\left(D^{*}\right)=0.302\pm 0.030\left(\text{stat}\right)\pm 0.011\left(\text{syst}\right)$, 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 technique.