Search for $CP$ Violation and Measurement of the Branching Fraction in the Decay $D^0\to K_S^0{K}_S^0$

In this work we study the decay $D^0\to K_S^0{K}_S^0$ using $921{\mathrm{fb}}^{-1}$ of data collected at or near the ${\rm Y}\left(4S\right)$ and ${\rm Y}\left(5S\right)$ resonances with the Belle detector at the KEKB asymmetric energy $e^{+}{e}^{-}$ collider. The measured time-integrated $CP$ asymmetry is $A_{CP}(D^0\to K_S^0{K}_S^0)=(-0.02\pm 1.53\pm 0.02\pm 0.17)\%$, and the branching fraction is $B(D^0\to K_S^0{K}_S^0)=\left(1.321\pm 0.023\pm 0.036\pm 0.044\right)\times {10}^{-4}$, where the first uncertainty is statistical, the second is systematic, and the third is due to the normalization mode ($D^0\to K_S^0{\pi }^0$). These results are furthermore more precise than previous measurements available for this mode. The $A_{CP}$ measurement is consistent with the standard model expectation.