Moritz Bauer ; Pablo Goldenzweig ; Nadia Toutounji ; Kevin Varvell

09 September 2021

Abstract: We present a reconstruction in early data of the semileptonic decay $B^+ \to \pi^0 \ell^+ \nu_\ell$, and first results of a reconstruction of the decays $B^+ \to \rho^0 \ell^+ \nu_\ell$ and $B^0 \to \rho^- \ell^+ \nu_\ell$ in a sample corresponding to 62.8\invfb of Belle II data using hadronic $B$-tagging via the full-event-interpretation algorithm. We determine the total branching fractions via fits to the distribution of the square of the missing mass, with \bfpiz{} = (8.29 $\pm$ 1.99$_{\mathrm{stat}}$ $\pm$ 0.46$_{\mathrm{sys}}$) $\times 10^{-5}$, \bfrhoz{} = ($9.26 \pm 6.33_{\mathrm{stat}}$ $\pm$ 0.38$_{\mathrm{sys}}$) $\times 10^{-5}$ and \bfrhoc = ($1.51 \pm 1.13_{\mathrm{stat}}$ $\pm$ 0.09$_{\mathrm{sys}}$) $\times 10^{-4}$. We also quote an updated branching fraction for the $B^0 \to \pi^- \ell^+ \nu_\ell$ decay, \bfpic{} = (1.47 $\pm$ 0.29$_{\mathrm{stat}}$ $\pm$ 0.05$_{\mathrm{sys}}$) $\times 10^{-4}$, based on the sum of the partial branching fractions in three bins of the momentum transfer to the leptonic system.

Keyword(s): Full-Event-Interpretation ; Hadronic Tagging ; Exclusive semi-leptonic