Observation of time-dependent CP violation in B0→η′K0 decays and improved measurements of CP asymmetries in B0→φK0, KS0KS0KS0 and B0→J/ψK0 decays

We present improved measurements of $CP$-violation parameters in ${B}^{0}\\ensuremath{\\rightarrow}\\ensuremath{\\phi}{K}^{0}$, ${\\ensuremath{\\eta}}^{\\ensuremath{'}}{K}^{0}$, ${K}_{S}^{0}{K}_{S}^{0}{K}_{S}^{0}$ decays based on a sample of $535\\ifmmode\\times\\else\\texttimes\\fi{}{10}^{6}$ $B\\overline{B}$ pairs collected at the $\\ensuremath{\\Upsilon}(4S)$ resonance with the Belle detector at the KEKB energy-asymmetric ${e}^{+}{e}^{\\ensuremath{-}}$ collider. We obtain $\\mathrm{sin}2{\\ensuremath{\\phi}}_{1}^{\\mathrm{eff}}=+0.64\\ifmmode\\pm\\else\\textpm\\fi{}0.10(\\mathrm{stat})\\ifmmode\\pm\\else\\textpm\\fi{}0.04(\\mathrm{syst})$ for ${B}^{0}\\ensuremath{\\rightarrow}{\\ensuremath{\\eta}}^{\\ensuremath{'}}{K}^{0}$, $+0.50\\ifmmode\\pm\\else\\textpm\\fi{}0.21(\\mathrm{stat})\\ifmmode\\pm\\else\\textpm\\fi{}0.06(\\mathrm{syst})$ for ${B}^{0}\\ensuremath{\\rightarrow}\\ensuremath{\\phi}{K}^{0}$, and $+0.30\\ifmmode\\pm\\else\\textpm\\fi{}0.32(\\mathrm{stat})\\ifmmode\\pm\\else\\textpm\\fi{}0.08(\\mathrm{syst})$ for ${B}^{0}\\ensuremath{\\rightarrow}{K}_{S}^{0}{K}_{S}^{0}{K}_{S}^{0}$ decays. We have observed $CP$ violation in the ${B}^{0}\\ensuremath{\\rightarrow}{\\ensuremath{\\eta}}^{\\ensuremath{'}}{K}^{0}$ decay with a significance of 5.6 standard deviations. We also perform an improved measurement of $CP$ asymmetries in ${B}^{0}\\ensuremath{\\rightarrow}J/\\ensuremath{\\psi}{K}^{0}$ decays and obtain $\\mathrm{sin}2{\\ensuremath{\\phi}}_{1}=+0.642\\ifmmode\\pm\\else\\textpm\\fi{}0.031(\\mathrm{stat})\\ifmmode\\pm\\else\\textpm\\fi{}0.017(\\mathrm{syst})$.