Measurement of the Branching Fraction for the Decay $\psi (3686)\to \phi K_S^0{K}_S^0$

Based on $(448.1\pm 2.9)\times {10}^6$ $\psi (3686)$ events collected with the BESIII detector operating at the BEPCII collider, the decay $\psi (3686)\to \phi K_S^0{K}_S^0$ is observed for the first time. Taking the interference between $\psi (3686)$ decay and continuum production into account, the branching fraction of this decay is measured to be $\mathcal{B}(\psi (3686)\to \phi K_S^0{K}_S^0)$ = $(3.53$ $\pm$ $0.20$ $\pm$ $0.21$)$\times {10}^{-5}$, where the first uncertainty is statistical and the second is systematic. Combining with the world average value for $\mathcal{B}(J\mathrm{/}\psi \to \phi K_S^0{K}_S^0)$, the ratio $\mathcal{B}(\psi (3686)\to \phi K_S^0{K}_S^0)\mathrm{/}\mathcal{B}(J\mathrm{/}\psi \to \phi K_S^0{K}_S^0)$ is determined to be $(6.0\pm 1.6)$%, which is suppressed relative to the 12% rule.