Branching fraction measurement of $J/\psi \to K_S{K}_L$ and search for $J/\psi \to K_S{K}_S$

Using a sample of $1.31\times {10}^{9}J/\psi$ events collected with the BESIII detector at the BEPCII collider, we study the decays of $J/\psi \to K_S{K}_L$ and $K_S{K}_S$. The branching fraction of $J/\psi \to K_S{K}_L$ is determined to be $\mathcal{B}(J/\psi \to K_S{K}_L)=(1.93\pm 0.01(\mathrm{stat})\pm 0.05(\mathrm{syst}))\times {10}^{-4}$, which significantly improves on previous measurements. No clear signal is observed for the $J/\psi \to K_S{K}_S$ process, and the upper limit at the 95% confidence level for its branching fraction is determined to be $\mathcal{B}(J/\psi \to K_S{K}_S)<1.4\times {10}^{-8}$, which improves on the previous searches by 2 orders in magnitude and reaches the order of the Einstein-Podolsky-Rosen expectation.

Figure 1

The distribution of $M_{{\pi }^{+}{\pi }^{-}}$. The (black) crosses are from data, and the (red) histogram represents the signal MC sample.

Figure 2

The momentum distribution of $K_S$ in the $e^{+}{e}^{-}$ rest frame. The (black) crosses are from data, and the (blue) solid line is the fit result. The (red) dash-dotted line is the signal, and the (green) dashed line is background.

Figure 3

The $K_S$ momentum distribution for data taken at $\sqrt{s}=3.080\mathrm{GeV}$. The (black) crosses are data, and the (blue) solid line is the fitting result. The (red) dash-dotted line corresponds to the signal, and the (green) dashed line represents the background.

Figure 4

The distribution of $K_S$ momentum in the $J/\psi$ rest frame. The (black) crosses are from data, and the (red) solid line is from the signal MC sample. The arrows indicate the $5\times {\sigma }_w$ selection region.