Determination of $|V_{cb}|$ using ${\overline{B}}^0\to D^{\ast +}{\ell }^{-}{\bar{\nu }}_{\ell }$ decays with Belle II

Abstract

We determine the CKM matrix-element magnitude $|V_{cb}|$ using ${\overline{B}}^0\to D^{\ast +}{\ell }^{-}{\bar{\nu }}_{\ell }$ 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 $\mathcal{B}({\overline{B}}^0\to D^{\ast +}{\ell }^{-}{\bar{\nu }}_{\ell })=(4.922\pm 0.023\pm 0.220)\%$. 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 $D^{\ast +}$ longitudinal polarization fractions. All results are compatible with lepton-flavor universality in the Standard Model.

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• FLAG Review 2024
• Global fit to $b\to c\tau \nu$ anomalies as of Spring 2024
• Determination of $|V_{cb}|$ using $B\to D\ell {\nu }_{\ell }$ Decays at Belle II
• On magnitudes of some CKM matrix elements
• A unified scheme for calculating the exclusive semi-leptonic decays of hadrons
• Lepton universality tests and searches for new physics in charged current decays at Belle II