Measurement of the semileptonic $bar{B}^0 to D^{*+} ell^{-} u_{ell}$ branching fraction with fully reconstructed $B$ meson decays and 34.6 fb$^{-1}$of Belle II data

We present a first measurement of the $bar{B^{0}} rightarrow D^{*+} ell^{-} u_{ell}$ branching fraction using fully reconstructed $B$ meson decays employing the Full Event Interpretation algorithm. Collision events corresponding to an integrated luminosity of lumi are analyzed, which were recorded by the Belle~II detector operated at the SuperKEKB accelerator complex. We measure$cal{B}(bar{B^{0}} rightarrow D^{*+} ell^{-} u_{ell}) =4.51 pm 0.41_{stat}pm0.27_{syst} pm0.45_{pi_s}$, with the first and second error denoting the statistical and systematic uncertainty, respectively, and the third dominant uncertainty is from the slow pion reconstruction efficiency.