A test of lepton flavor universality with a measurement of $R(D^{\ast })$ using hadronic $B$ tagging at the Belle II experiment

Abstract

The ratio of branching fractions $R(D^{\ast })=\mathcal{B}(\overline{B}\to D^{\ast }\ddot{I}{„}^{-}\overline{\hat{I}}{½}_{\ddot{I}}„)$/$\mathcal{B}(\overline{B}\to D^{\ast }{\ell }^{-}\overline{\hat{I}}{½}_{\ell })$, where $\ell$ is an electron or muon, is measured using a Belle~II data sample with an integrated luminosity of $189{\mathrm{fb}}^{-1}$ at the SuperKEKB asymmetric-energy $e^{+}{e}^{-}$ collider. Data is collected at the $\hat{I}\yen (4S)$ resonance, and one $B$ meson in the $\hat{I}\yen (4S)\to B\overline{B}$ decay is fully reconstructed in hadronic decay modes. The accompanying signal $B$ meson is reconstructed as $\overline{B}\to D^{\ast }\ddot{I}{„}^{-}\overline{\hat{I}}{½}_{\ddot{I}}„$ using leptonic $\ddot{I}„$ decays. The normalization decay, $\overline{B}\to D^{\ast }{\ell }^{-}\overline{\hat{I}}{½}_{\ell }$, where $\ell$ is an electron or muon, produces the same observable final state particles. The ratio of branching fractions is extracted in a simultaneous fit to two signal-discriminating variables in both channels and yields $R(D^{\ast })=0.262{}_{-0.039}^{+0.041}(\mathrm{stat}){}_{-0.032}^{+0.035}(\mathrm{syst})$. This result is consistent with the current world average and with standard model predictions.

16 pages, 17 figures, submitted to PRD