Test of lepton flavor universality with measurements of $R(D^{+})$ and $R(D^{\ast +})$ using semileptonic $B$ tagging at the Belle II experiment

We report measurements of the ratios of branching fractions $\mathcal{R}(D^{(\ast )+})=\frac{\mathcal{B}(\bar{B}{}^0\to D^{(\ast )+}{\tau }^{-}{\bar{\nu }}_{\tau })}{\mathcal{B}(\bar{B}{}^0\to D^{(\ast )+}{\mathrm{\ell }}^{-}{\bar{\nu }}_{\mathrm{\ell }})}$, where $\mathrm{\ell }$ denotes either an electron or a muon. These ratios test the universality of the charged-current weak interaction. The results are based on a $365{\mathrm{f}\mathrm{b}}^{-1}$ data sample collected with the Belle II detector at the SuperKEKB $e^{+}{e}^{-}$ collider, which operates at a center-of-mass energy corresponding to the $\mathrm{{\rm Y}}(4S)$ resonance, just above the threshold for $B\bar{B}$ production. Signal candidates are reconstructed by selecting events in which the companion $B$ meson from the $\mathrm{{\rm Y}}(4S)\to B\bar{B}$ decay is identified in semileptonic modes. The $\tau$ lepton is reconstructed via its leptonic decays. We obtain $\mathcal{R}(D^{+})=0.418\pm 0.074(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})\pm 0.051(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})$ and $\mathcal{R}(D^{\ast +})=0.306\pm 0.034(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})\pm 0.018(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})$, which are consistent with world average values. Accounting for the correlation between them, these values differ from the Standard Model expectation by a collective significance of $1.7$ standard deviations.