Florian Bernlochner ; Lu Cao ; Daniel Dorner ; Philipp Horak ; Chaoyi Lyu ; Minakshi Nayak ; Christoph Schwanda ; Abi Soffer

05 August 2020

Abstract: We report measurements of the $\bar B^0\to D^{*+}\ell^-\bar\nu_\ell$ and $B^-\to D^{0}\ell^-\bar\nu_\ell$ processes using \mbox{34.6 fb$^{-1}$} of collision events recorded by the Belle~II experiment at the SuperKEKB asymmetric-energy $e^+ e^-$ collider. For the $B^-\to D^{0}\ell^-\bar\nu_\ell$ channel, we present first studies that isolate this decay from other semileptonic processes and backgrounds. We report a measurement of the $\bar B^0\to D^{*+}\ell^-\bar\nu_\ell$ branching fraction and obtain $\mathcal{B}(\bar B^0\to D^{*+}\ell^-\bar\nu_\ell) = \left(4.60 \pm 0.05_{\mathrm{stat}}\pm 0.17_{\mathrm{syst}} \pm 0.45_{\pi_s}\right)$ in agreement with the world average. Here the uncertainties are statistical, systematic, and related to slow pion reconstruction, respectively. The systematic uncertainties are limited by the statistics of auxiliary measurements and will improve in the future. We also report differential branching fractions in five bins of the hadronic recoil parameter $w$ for $\bar B^0\to D^{*+}\ell^-\bar\nu_\ell$, unfolded to account for resolution and efficiency effects.

Keyword(s): Semileptonic decays, Vcb