Measurement of Time-Dependent $CP$ Violation in $B^0to etaK^0$ Decays

We present a measurement of the time-dependent $CP$ violation parameters in $B^0toetaK^0$ decays. The measurement is based on the full data sample containing $772times 10^6$ $Bbar{B}$ pairs collected at the $Upsilon(4S)$ resonance using the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. The measured values of the mixing-induced and direct $CP$ violation parameters are: begin{align} sin 2 phi^{rm eff}_1 &= +0.68pm 0.07 pm 0.03, onumber mathcal{A}_{etaK^0} &= +0.03pm 0.05pm 0.04, onumber end{align} where the first uncertainty is statistical and the second is systematic. The values obtained are the most accurate to date. Furthermore, these results are consistent with our previous measurements and with the world-average value of $sin 2phi_1$ measured in $B^0to J/psi K^0$ decays.}