Measurement of the $B^0\to D^{\ast -}{\ell }^{+}\hat{I}{½}_{\ell }$ branching ratio and $|V_{cb}|$ with a fully reconstructed accompanying $B$ meson in 2019-2021 Belle II data

Abstract

We present a measurement of the $B^0\to D^{\ast -}{\ell }^{+}\hat{I}{½}_{\ell }$ ($\ell =e,\hat{I}¼$) branching ratio and of the CKM parameter $|V_{cb}|$ using signal decays accompanied by a fully reconstructed $B$ meson. The Belle II data set of electron-positron collisions at the $\hat{I}\yen (4S)$ resonance, corresponding to 189.3fb${}^{-1}$ of integrated luminosity, is analyzed. With the Caprini-Lellouch-Neubert form factor parameterization, the parameters $\hat{I}{\cdot }_{\mathrm{EW}}F(1)|V_{cb}|$ and $\ddot{I}{}^2$ are extracted, where $\hat{I}{\cdot }_{\mathrm{EW}}$ is an electroweak correction, $F(1)$ is a normalization factor and $\ddot{I}{}^2$ is a form factor shape parameter. We reconstruct 516 signal decays and thereby obtain $\mathcal{B}(B^0\to D^{\ast -}{\ell }^{+}\hat{I}{½}_{\ell })=\left(5.27\pm 0.22\left(\mathrm{stat}\right)\pm 0.38\left(\mathrm{syst}\right)\right)\%$, $\hat{I}{\cdot }_{EW}F(1)|V_{cb}|\times 10^3=34.6\pm 1.8\left(\mathrm{stat}\right)\pm 1.7\left(\mathrm{syst}\right)$, and $\ddot{I}{}^2=0.94\pm 0.18\left(\mathrm{stat}\right)\pm 0.11\left(\mathrm{syst}\right)$.