Measurement of the Time-Integrated CP Asymmetry in D^0→ K^0_S K^0_S Decay at Belle and Belle II

We measure the time-integrated CP asymmetry, A _CP , in D^0 → K^0_S K^0_S decays reconstructed in e^+e^-→ cc̅ events collected by the Belle and Belle II experiments with the data sample corresponding to an integrated luminosities of 980 fb^-1 and 427 fb^-1 , respectively. The D^0 decays are required to come from the flavor-conserving D^*+→ D^0π ^+ decay ( D^*+ tag) to determine the charm flavor at production time. A control sample of D^*+ -tagged D^0→ K^+ K^- is used to correct for production and detection asymmetries. The result, A_CP(D^0→ K^0_S K^0_S ) = (-1.4 ± 1.3(stat) ± 0.1(syst))% , is consistent with previous determinations and with CP symmetry.