We determine the CKM matrix-element magnitude $|V_{ cb}|$ using $\overline{ B}^0\to D^{ *+}\ell^-\bar\nu_\ell$ decays reconstructed in $189 \, \mathrm{ fb}^{ -1}$ of collision data collected by the Belle II experiment, located at the SuperKEKB $e^+e^-$ collider. Partial decay rates are reported as functions of the recoil parameter $w$ and three decay angles separately for electron and muon final states. We obtain $|V_{ cb}|$ using the Boyd-Grinstein-Lebed and Caprini-Lellouch-Neubert parametrizations, and find $|V_{ cb}|_\mathrm{ BGL}=(40.57\pm 0.31 \pm 0.95\pm 0.58)\times 10^{ -3}$ and $|V_{ cb}|_\mathrm{ CLN}=(40.13 \pm 0.27 \pm 0.93\pm 0.58 )\times 10^{ -3}$ with the uncertainties denoting statistical components, systematic components, and components from the lattice QCD input, respectively. The branching fraction is measured to be ${ \cal B}(\overline{ B}^0\to D^{ *+}\ell^-\bar\nu_\ell)=(4.922 \pm 0.023 \pm 0.220)\%$. The ratio of branching fractions for electron and muon final states is found to be $0.998 \pm 0.009 \pm 0.020$. In addition, we determine the forward-backward angular asymmetry and the $D^{ *+}$ longitudinal polarization fractions. All results are compatible with lepton-flavor universality in the Standard Model.
Categories: hep-ex