Observation of Time-Dependent $CP$ Violation in $B^0\to {\eta }^{\prime }{K}^0$ Decays and Improved Measurements of $CP$ Asymmetries in $B^0\to \phi K^0$, $K_S^0{K}_S^0{K}_S^0$ and $B^0\to J/\psi K^0$ Decays

We present improved measurements of $CP$-violation parameters in $B^0\to \varphi K^0$, ${\eta }^{\prime }{K}^0$, $K_S^0{K}_S^0{K}_S^0$ decays based on a sample of $535\times {10}^6$ $B\overline{B}$ pairs collected at the ${\rm Y}(4S)$ resonance with the Belle detector at the KEKB energy-asymmetric $e^{+}{e}^{-}$ collider. We obtain $\sin 2{\varphi }_1^{\mathrm{eff}}=+0.64\pm 0.10(\mathrm{stat})\pm 0.04(\mathrm{syst})$ for $B^0\to {\eta }^{\prime }{K}^0$, $+0.50\pm 0.21(\mathrm{stat})\pm 0.06(\mathrm{syst})$ for $B^0\to \varphi K^0$, and $+0.30\pm 0.32(\mathrm{stat})\pm 0.08(\mathrm{syst})$ for $B^0\to K_S^0{K}_S^0{K}_S^0$ decays. We have observed $CP$ violation in the $B^0\to {\eta }^{\prime }{K}^0$ decay with a significance of 5.6 standard deviations. We also perform an improved measurement of $CP$ asymmetries in $B^0\to J/\psi K^0$ decays and obtain $\sin 2{\varphi }_1=+0.642\pm 0.031(\mathrm{stat})\pm 0.017(\mathrm{syst})$.