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

We report a measurement of the ratios of branching fractions $$\mathcal{R}$(D^{()}) = {\cal B}(\bar{B} \to D^{()} \tau^- \bar{\nu}{\tau})/{\cal B}(\bar{B} \to D^{(*)} \ell^- \bar{\nu}{\ell})$where$\ell$ denotes an electron or a muon. The results are based on a data sample containing $772\times {10}^6$ $B\bar{B}$ events recorded at the $\mathrm{{\rm Y}}(4S)$ resonance with the Belle detector at the KEKB $e^{+}{e}^{-}$ collider. The tag-side B meson is reconstructed in a semileptonic decay mode, and the signal-side $\tau$ is reconstructed in a purely leptonic decay. The results are $\mathcal{R}(D)=0.307\pm 0.037\pm 0.016$ and $$\mathcal{R}$(D^*)=0.283 \pm 0.018 \pm 0.014$, where the first uncertainties are statistical and the second are systematic. These results are in agreement with the Standard Model predictions within 0.2 and 1.1 standard deviations, respectively.