Studies of the semileptonic $bar B^0to D^{*+}ell^-bar u_ell$ and $B^-to D^{0}ell^-bar u_ell$ decay processes with 34.6 fb$^{-1}$ of Belle II data

We report measurements of the $bar{B}^0 to D^{*+} ell^{-} bar{ u}_l$ and $B^- to D^{0} ell^{-} bar{ u}_l$ processes using 34.6 fb$^{-1}$ of collision events recorded by the Belle II experiment at the SuperKEKB asymmetric-energy $e^+ e^-$ collider. For the $B^-to D^{0}ell^-bar u_ell$ channel, we present first studies that isolate this decay from other semileptonic processes and backgrounds. We report a measurement of the $bar{B}^0 to D^{*+} ell^{-} bar{ u}_l$ branching fraction and obtain ${cal B}(bar{B}^0 to D^{*+} ell^{-} bar{ u}_l) = left(4.60 pm 0.05_{mathrm{stat}}pm0.17_{mathrm{syst}} pm 0.45_{pi_s}right) %$, 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 $bar{B}^0 to D^{*+} ell^{-} bar{ u}_l$, unfolded to account for resolution and efficiency effects.