Search for Dark Particles in $K^0_L \to \gamma X$ at the KOTO Experiment

TL;DR

This study searched for dark particles in $K^0_L$ decays at the KOTO experiment, setting upper limits on the decay's branching ratio across a range of dark particle masses, with no evidence of such particles found.

Contribution

First search for dark particles in $K^0_L o ext{gamma} X$ decays at KOTO, establishing new upper limits on the branching ratio for various dark particle masses.

Findings

No significant excess over background observed.

Upper limits on branching ratios range from $3.4 imes 10^{-7}$ to $10^{-3}$.

Constraints placed on dark photon models within the studied mass range.

Abstract

We report a search for an invisible particle $X$ in the decay $K^0_L\rightarrow \gamma X$ ($X \to \text{invisible}$), 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 $K^0_L\rightarrow \gamma X$ were set for the $X$ mass range $0 \leq m_X \leq 425$ MeV/$c^2$. For massless $X$, the upper limit was $3.4\times10^{-7}$ at the $90\%$ confidence level, while for massive $X$, the upper limits in the searched mass region ranged from $O(10^{-7})$ to $O(10^{-3})$.