Measurement of $CP$ asymmetries in $B^0 \to K^0_S \pi^0 \gamma$ decays at Belle II

TL;DR

This paper reports precise measurements of time-dependent $CP$ asymmetries in $B^0 o K^0_S o ext{pi}^0 ext{gamma}$ decays using Belle II data, testing the Standard Model's predictions for $CP$ violation.

Contribution

First measurement of $CP$ asymmetries in $B^0 o K^0_S ext{pi}^0 ext{gamma}$ decays at Belle II with improved precision, covering different $K^0_S ext{pi}^0$ invariant mass regions.

Findings

Results are consistent with Standard Model predictions.

Improved measurement precision over previous experiments.

No evidence of new physics beyond the Standard Model.

Abstract

We report measurements of time-dependent $CP$ asymmetries in $B^0 \to K^0_S \pi^0 \gamma$ decays based on a data sample of $(388\pm6)\times10^6$ $B\bar{B}$ events collected at the $\Upsilon(4S)$ resonance with the Belle II detector. The Belle II experiment operates at the SuperKEKB asymmetric-energy $e^+e^-$ collider. We measure decay-time distributions to determine $CP$-violating parameters $S$ and $C$. We determine these parameters for two ranges of $K^0_S \pi^0$ invariant mass: $m(K^0_S \pi^0)\in (0.8, 1.0)$ $GeV/c^2$, which is dominated by $B^0 \to K^{*0} (\to K^0_S \pi^0) \gamma$ decays, and a complementary region $m(K^0_S \pi^0)\in (0.6, 0.8)\cup(1.0, 1.8)$ $GeV/c^2$. Our results have improved precision as compared to previous measurements and are consistent with theory predictions.