Measurement of decay-time dependent $CP$ violation in $B^0\to K_S^0{K}_S^0{K}_S^0$ using 2019--2021 Belle II data

We report a measurement of decay-time dependent $CP$-violating parameters in $B^0\to K_S^0{K}_S^0{K}_S^0$ decays. We use $(198.0\pm 3.0)\times {10}^{6}B\bar{B}$ pairs collected at the $\mathrm{{\rm Y}}(4S)$ resonance with the Belle II detector at the SuperKEKB asymmetric-energy $e^{+}{e}^{-}$ collider. The observed mixing-induced and direct $CP$ violation parameters are $\mathcal{S}=-1.86{}_{-0.46}^{+0.91}(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})\pm 0.09(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})$ and $\mathcal{A}=-0.22{}_{-0.27}^{+0.30}(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})\pm 0.04(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})$, respectively.