Search for $J\mathrm{/}\psi \to K_S^0{K}_S^0$ and $\psi (3686)\to K_S^0{K}_S^0$

Using data samples of $(10087\pm 44)\times {10}^6$ $J\mathrm{/}\psi$ events and $(2712.4\pm 14.3)\times {10}^6$ $\psi (3686)$ events collected with the BESIII detector at the BEPCII collider, we search for the CP violating decays $J\mathrm{/}\psi \to K_S^0{K}_S^0$ and $\psi (3686)\to K_S^0{K}_S^0$. No significant signals are observed over the expected background yields. The upper limits on their branching fractions are set as $\mathcal{B}(J\mathrm{/}\psi \to K_S^0{K}_S^0)<4.7\times {10}^{-9}$ and $\mathcal{B}(\psi (3686)\to K_S^0{K}_S^0)<1.1\times {10}^{-8}$ at the 90% confidence level. These results improve the previous limits by a factor of three for $J\mathrm{/}\psi \to K_S^0{K}_S^0$ and two orders of magnitude for $\psi (3686)\to K_S^0{K}_S^0$.