Determination of $|V_{cb}|$ from $B\to D\ell\nu$ decays using 2019-2021 Belle II data

TL;DR

This paper measures the CKM matrix element |V_{cb}| using B→Dℓν decays with Belle II data, employing a novel method to reconstruct B meson kinematics and fitting form factors with lattice QCD inputs.

Contribution

It introduces a new approach to determine |V_{cb}| from semileptonic B decays using Belle II data and the diamond-frame method for improved kinematic reconstruction.

Findings

|V_{cb}| measured as (38.53 ± 1.15)×10^{-3}

Utilizes BGL form factor parameterization and lattice QCD inputs

Achieves a precise determination with combined theoretical and experimental uncertainties.

Abstract

We present a determination of the magnitude of the Cabibbo-Kobayashi-Maskawa (CKM) matrix element $V_{cb}$ using $B\to D\ell\nu$ decays. The result is based on $e^+e^-\to\Upsilon(4S)$ data recorded by the Belle II detector corresponding to 189.2/fb of integrated luminosity. The semileptonic decays $B^0\to D^-(\to K^+\pi^-\pi^-)\ell^+\nu_\ell$ and $B^+\to\bar D^0(\to K^+\pi^-)\ell^+\nu_\ell$ are reconstructed, where $\ell$ is either electron or a muon. The second $B$ meson in the $\Upsilon(4S)$ event is not explicitly reconstructed. Using the diamond-frame method, we determine the $B$ meson four-momentum and thus the hadronic recoil. We extract the partial decay rates as functions of $w$ and perform a fit to the decay form-factor and the CKM parameter $|V_{cb}|$ using the BGL parameterization of the form factor and lattice QCD input from the FNAL/MILC and HPQCD collaborations. We obtain $\eta_{EW}|V_{cb}|=(38.53\pm 1.15)\times 10^{-3}$, where $\eta_{EW}$ is an electroweak correction, and the error accounts for theoretical and experimental sources of uncertainty.