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

We report a measurement of decay-time dependent charge-parity ($CP$) asymmetries in B⁰ → K⁰S K⁰S K⁰S decays. We use 387 × 10⁶ BB̄ pairs collected at the Υ(4S) resonance with the Belle II detector at the SuperKEKB asymmetric-energy electron-positron collider. We reconstruct 220 signal events and extract the $CP$-violating parameters S and C from a fit to the distribution of the decay-time difference between the two B mesons. The resulting confidence region is consistent with previous measurements in B⁰ → K⁰S K⁰S K⁰S and B⁰ → (cc̄)K⁰ decays, and with predictions based on the standard model.