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We report a measurement of the ratios of branching fractions $\mathcal{R}(D)= {\cal B}(\bar{B} \to D \tau^- \bar{\nu}_{\tau})/{\cal B}(\bar{B} \to D \ell^-\bar{\nu}_{\ell})$ and $\mathcal{R}(D^{\ast}) = {\cal B}(\bar{B} \to D^* \tau^-\bar{\nu}_{\tau})/{\cal B}(\bar{B} \to D^* \ell^- \bar{\nu}_{\ell})$, where$\ell$ denotes an electron or a muon. The results are based on a data samplecontaining $772\times10^6$ $B\bar{B}$ events recorded at the $\Upsilon(4S)$resonance with the Belle detector at the KEKB $e^+ e^-$ collider. The analysisutilizes a method where the tag-side $B$ meson is reconstructed in asemileptonic decay mode, and the signal-side $\tau$ is reconstructed in apurely leptonic decay. The measured values are $\mathcal{R}(D)= 0.307 \pm 0.037\pm 0.016$ and $\mathcal{R}(D^{\ast})= 0.283 \pm 0.018 \pm 0.014$, where thefirst uncertainties are statistical and the second are systematic. Theseresults are in agreement with the Standard Model predictions within $0.2$ and$1.1$ standard deviations, respectively, while their combination agrees withthe Standard Model predictions within $1.2$ standard deviations.

Measurement of $\mathcal{R}(D)$ and $\mathcal{R}(D^{\ast})$ with a semileptonic tagging method