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

The experimental results on the ratios of branching fractions $mathcal{R}(D) = {cal B}(bar{B} to D tau^- bar{ u}_{tau})/{cal B}(bar{B} to D ell^- bar{ u}_{ell})$ and $mathcal{R}(D^*) = {cal B}(bar{B} to D^* tau^- bar{ u}_{tau})/{cal B}(bar{B} to D^* ell^- bar{ u}_{ell})$, where $ell$ denotes an electron or a muon, show a long-standing discrepancy with the Standard Model predictions, and might hint to a violation of lepton flavor universality. We report a new simultaneous measurement of $mathcal{R}(D)$ and $mathcal{R}(D^*)$, based on a data sample containing $772 times 10^6$ $Bbar{B}$ events recorded at the $Upsilon(4S)$ resonance with the Belle detector at the KEKB $e^+ e^-$ collider. In this analysis 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 measured values 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 $0.2$, $1.1$ and $0.8$ standard deviations for $mathcal{R}(D)$, $mathcal{R}(D^*)$ and their combination, respectively. This work constitutes the most precise measurements of $mathcal{R}(D)$ and $mathcal{R}(D^*)$ performed to date as well as the first result for $mathcal{R}(D)$ based on a semileptonic tagging method.