The Search for $K_L \rightarrow \pi^0\pi^0\gamma\gamma$ and $K_L\rightarrow \pi^0\pi^0X$ where $X\rightarrow 2\gamma$ at the KOTO Experiment

Abstract

We performed searches for $K_L\rightarrow \pi^0\pi^0X$ where $X$ may be an axion-like particle which promptly decays to two photons, and the first search for $K_L \rightarrow \pi^0\pi^0\gamma\gamma$ at the KOTO experiment using data taken in 2021. The search is performed for $X$ mass in the range of 160$\unicode{x2013}$220 MeV/$c^2$. Three events were observed in the signal region, with two events near an $X$ mass of 177 MeV/$c^2$. This result led to a range of upper limits on the branching ratio, BR($K_L\rightarrow \pi^0\pi^0X$) $< (1\unicode{x2013}20) \times 10^{-7}$ at the 95% confidence level (C.L.). No events were observed for the analysis of $K_L \rightarrow \pi^0\pi^0\gamma\gamma$, setting an upper limit on the branching ratio, BR($K_L \rightarrow \pi^0\pi^0\gamma\gamma$) $< 1.69 \times 10^{-6}$ at the 95% C.L.