We present a determination of the Cabibbo-Kobayashi-Maskawa matrix element $|V_{ cb}|$ from the decay $B\to D\ell\nu_\ell$ using a $365~\mathrm{ fb}^{ -1}$ $e^+e^-\to\Upsilon(4S)\to B\bar B$ data sample recorded by the Belle II experiment at the SuperKEKB collider. The semileptonic decay of one $B$ meson is reconstructed in the modes $B^0\to D^-(\to K^+\pi^-\pi^-)\ell^+\nu_\ell$ and $B^+\to \bar D^0(\to K^+\pi^-)\ell^+\nu_\ell$, where $\ell$ denotes either an electron or a muon. Charge conjugation is implied. The second $B$ meson in the $\Upsilon(4S)$ event is not reconstructed explicitly. Using an inclusive reconstruction of the unobserved neutrino momentum, we determine the recoil variable $w=v_B\cdot v_D$, where $v_B$ and $v_D$ are the 4-velocities of the $B$ and $D$ mesons. We measure the total decay branching fractions to be $\mathcal{ B}(B^0\to D^-\ell^+\nu_\ell)=(2.06 \pm 0.05\,(\mathrm{ stat.}) \pm 0.10\,(\mathrm{ sys.}))\%$ and $\mathcal{ B}(B^+\to\bar D^0\ell^+\nu_\ell)=(2.31 \pm 0.04\,(\mathrm{ stat.}) \pm 0.09\,(\mathrm{ sys.}))\%$. We probe lepton flavor universality by measuring $\mathcal{ B}(B\to De\nu_e)/\mathcal{ B}(B\to D\mu\nu_\mu)=1.020 \pm 0.020\,(\mathrm{ stat.})\pm 0.022\,(\mathrm{ sys.})$. Fitting the partial decay branching fraction as a function of $w$ and using the average of lattice QCD calculations of the $B\to D$ form factor, we obtain $ |V_{ cb}|=(39.2\pm 0.4\,(\mathrm{ stat.}) \pm 0.6\,(\mathrm{ sys.}) \pm 0.5\,(\mathrm{ th.})) \times 10^{ -3}$.
Categories: hep-ex