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

Abstract

We report measurements of time-dependent $CP$ asymmetries in $B^0 \to K^0_S π^0 γ$ decays based on a data sample of $(388\pm6)\times10^6$ $B\bar{B}$ events collected at the $Υ(4S)$ 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 $K^0_S π^0$ invariant mass: $m(K^0_S π^0)\in (0.8, 1.0)$ $GeV/c^2$, which is dominated by $B^0 \to K^{*0} (\to K^0_S π^0) γ$ decays, and a complementary region $m(K^0_S π^0)\in (0.6, 0.8)\cup(1.0, 1.8)$ $GeV/c^2$. Our results have improved precision as compared to previous measurements and are consistent with theory predictions.

Figures (2)

• Figure 1: Distributions of $M_{\text{bc}}$ (left) and $\Delta E$ (right) for MR1 (top) and MR2 (bottom), with fit results overlaid. The $M_{\text{bc}}$ ($\Delta E$) distribution corresponds to the $\Delta E$ ($M_{\text{bc}}$) signal-enhanced region.
• Figure 2: $\Delta t$ distributions for MR1 (left) and MR2 (right) with the fit result superimposed. The asymmetry $[N^{}_s(B^0\xspace_{\rm tag}\xspace)-N^{}_s(\overline{ B}{}\xspace^0\xspace_{\rm tag}\xspace)]/[N^{}_s(B^0\xspace_{\rm tag}\xspace)+N^{}_s(\overline{ B}{}\xspace^0\xspace_{\rm tag}\xspace)]$ is calculated in each $\Delta t$ bin using $_{s}\mathcal{P}lot$Pivk:2004ty and plotted in the bottom panels along with the fit result.