Exclusive $B^0 \to \pi^- \ell^+ \nu_\ell$ Decays with Hadronic Full Event Interpretation Tagging in 34.6 fb$^{-1}$ of Belle II Data

TL;DR

This paper reports the first observation of the decay $B^0 o ext{pi}^- ext{l}^+ u_ ext{l}$ using Belle II data, achieving a significance of 5.69 sigma and measuring its branching fraction.

Contribution

It demonstrates the successful application of hadronic full event interpretation tagging to observe this decay mode in Belle II data.

Findings

Observed 21 signal events with 5.69 sigma significance.

Measured branching fraction of (1.58 ± 0.43_stat ± 0.07_sys) × 10^{-4}.

Validated the effectiveness of hadronic tagging in Belle II analyses.

Abstract

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$_{\mathrm{sys}}$) $\times 10^{-4}$.