Measurement of $CP$ violation in $B^0\to K_S^0\ddot{I}{€}^0$ decays at Belle II

Abstract

We report a measurement of the $CP$-violating parameters $A$ and $S$ in $B^0\to K_S^0\ddot{I}{€}^0$ decays at Belle II using a sample of $387\times 10^6$ $B\bar{B}$ events recorded in $e^{+}{e}^{-}$ collisions at a center-of-mass energy corresponding to the $\hat{I}\yen (4S)$ resonance. These parameters are determined by fitting the proper decay-time distribution of a sample of 415 signal events. We obtain $A=0.04_{-0.14}^{+0.15}\pm 0.05$ and $S=0.75_{-0.23}^{+0.20}\pm 0.04$, where the first uncertainties are statistical and the second are systematic.