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Abstract

We report a measurement of the ratio $\mathcal{R}({D}^{*})=\mathcal{B}({\overline{B}}^{0}\ensuremath{\rightarrow}{D}^{*+}{\ensuremath{\tau}}^{\ensuremath{-}}{\overline{\ensuremath{\nu}}}_{\ensuremath{\tau}})/\mathcal{B}({\overline{B}}^{0}\ensuremath{\rightarrow}{D}^{*+}{\ensuremath{\ell}}^{\ensuremath{-}}{\overline{\ensuremath{\nu}}}_{\ensuremath{\ell}})$, where $\ensuremath{\ell}$ denotes an electron or a muon. The results are based on a data sample containing $772\ifmmode\times\else\texttimes\fi{}1{0}^{6}\text{ }\text{ }B\overline{B}$ pairs recorded at the $\mathrm{\ensuremath{\Upsilon}}(4S)$ resonance with the Belle detector at the KEKB ${e}^{+}{e}^{\ensuremath{-}}$ collider. We select a sample of ${B}^{0}{\overline{B}}^{0}$ pairs by reconstructing both $B$ mesons in semileptonic decays to ${D}^{*\ensuremath{\mp}}{\ensuremath{\ell}}^{\ifmmode\pm\else\textpm\fi{}}$. We measure $\mathcal{R}({D}^{*})=0.302\ifmmode\pm\else\textpm\fi{}0.030(\text{stat})\ifmmode\pm\else\textpm\fi{}0.011(\text{syst})$, which is within $1.6\ensuremath{\sigma}$ of the Standard Model theoretical expectation, where the standard deviation $\ensuremath{\sigma}$ includes systematic uncertainties. We use this measurement to constrain several scenarios of new physics in a model-independent approach.