Measurement of $CP$ asymmetries in B⁰ → K⁰S π⁰ γ decays at Belle II

We report measurements of time-dependent $CP$ asymmetries in B⁰ → K⁰S π⁰ γ decays based on a data sample of (388±6)×10⁶ BB̄ 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 K⁰S π⁰ invariant mass: m(K⁰S π⁰)∈ (0.8, 1.0) GeV/c², which is dominated by B⁰ → K*0 (→ K⁰S π⁰) γ decays, and a complementary region m(K⁰S π⁰)∈ (0.6, 0.8)∪(1.0, 1.8) GeV/c². Our results have improved precision as compared to previous measurements and are consistent with theory predictions.