We present the results of the re-discovery of the decay $B^0 \to \pi^- \ell^+ \nu_\ell$ in 34.6 fb$^{ -1}$ of Belle II data using hadronic $B$-tagging via the Full Event Interpretation algorithm. We observe 21 signal events on a background of 155 in a fit to the distribution of the square of the missing mass, $M_{ \mathrm{ miss}}^2$, with a significance of 5.69$\sigma$, and determine a total branching fraction of (1.58 $\pm$ 0.43$_{ \mathrm{ stat}}$ $\pm$ 0.07$_{ \mathrm{ sys}}$) $\times 10^{ -4}$.
Categories: hep-ex