Measurements of branching fractions and CP-violating charge asymmetries in charmless $B$ decays reconstructed in 2019--2020 Belle~II data

TL;DR

This paper presents the first measurements of branching fractions and CP asymmetries in charmless B decays at Belle II, using 2019-2020 data, demonstrating detector performance comparable to Belle and establishing a foundation for future studies.

Contribution

It provides the first measurements of these quantities at Belle II, validating the detector's capabilities and analysis methods for charmless B decays.

Findings

Results are consistent with previous measurements.

Detector performance is comparable to Belle.

Establishes a baseline for future precision studies.

Abstract

We report on first measurements of branching fractions~($\mathcal{B}$) and CP-violating charge asymmetries~($\mathcal{A}$) in charmless $B$ decays at Belle~II. We use a sample of electron-positron collisions collected in 2019 and 2020 at the $\Upsilon(4S)$ resonance and corresponding to $34.6$\,fb$^{-1}$ of integrated luminosity. We use simulation to determine optimized event selections. The $\Delta E$ distributions of the resulting samples, restricted in $M_{\rm bc}$, are fit to determine signal yields. Signal yields are corrected for efficiencies determined from simulation and control data samples to obtain branching fractions and CP-violating asymmetries for flavour-specific channels. These are the first measurements in charmless decays reported by Belle~II. Results are compatible with known determinations and show detector performance comparable with the best Belle results offering a reliable basis to assess projections for future reach.