Measurement of $CP$ asymmetries in $B^0\to K_S^0{K}_S^0{K}_S^0$ decays at Belle II

We report a measurement of decay-time dependent charge-parity ($CP$) asymmetries in $B^0\to K_S^0{K}_S^0{K}_S^0$ decays. We use $387\times {10}^{6}B\bar{B}$ pairs collected at the $\mathrm{{\rm Y}}(4S)$ 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 $B^0\to K_S^0{K}_S^0{K}_S^0$ and $B^0\to (c\bar{c})K^0$ decays, and with predictions based on the standard model.