Search for Dark Particles in $K_L^0\to {\hat{I}}^{3}X$ at the KOTO Experiment

Abstract

We report a search for an invisible particle $X$ in the decay $K_L^0\to {\hat{I}}^{3}X$ ($X\to \text{invisible}$), where $X$ can be interpreted as a massless or massive dark photon. No evidence for $X$ was found, based on 13 candidate events consistent with a predicted background of $12.66\pm 4.42_{\text{stat.}}\pm 2.13_{\text{syst.}}$ events. Upper limits on the branching ratio of $K_L^0\to {\hat{I}}^{3}X$ were set for the $X$ mass range $0\le m_X\le 425$ MeV/$c^2$. For massless $X$, the upper limit was $3.4\times 10^{-7}$ at the $90\%$ confidence level, while for massive $X$, the upper limits in the searched mass region ranged from $O(10^{-7})$ to $O(10^{-3})$.