Philipp Horak ; Christoph Schwanda

07 October 2021

Abstract: We report a measurement of the branching fraction of the semileptonic decay \bdlnu (and its charge conjugate) using \mbox{$62.8~\text{fb}^{-1}$} of $\Upsilon(4S)\to B\bar B$~data recorded by the Belle II experiment at the SuperKEKB asymmetric-energy $e^+ e^-$ collider. The neutral charm meson is searched for in the decay mode $D^0\to K^-\pi^+$ and combined with a properly charged identified lepton (electron or muon) to reconstruct this decay. No reconstruction of the second $B$~meson in the $\Upsilon(4S)$~event is performed. We obtain ${\cal B}(B^-\to D^0 \ell^-\bar\nu_\ell) = (2.293\pm 0.053_\mathrm{stat}\pm 0.084_\mathrm{syst})\%$, in agreement with the world average of this decay. We also determine the ratio of the electron to muon branching fractions to be $R(e/\mu) = 1.044 \pm 0.046_\mathrm{stat} \pm 0.034_\mathrm{syst}$ and observe no deviation from lepton universality.