Search for $ J/\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/\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/\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/\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/\psi \to {K}_{S}^{0}{K}_{S}^{0}$ and two orders of magnitude for $ \psi (3686)\to {K}_{S}^{0}{K}_{S}^{0}$.