Search for the Pair Production of Dark Particles $X$ with $K_L^0 \to XX$, $X \to \gamma\gamma$

TL;DR

This paper reports the first search for dark particle pair production via $K_L^0$ decays into two photons, setting new upper limits on the branching fractions in specific mass ranges using KOTO data.

Contribution

It introduces the first experimental search for $K_L^0 o XX$ with $X o ext{photons}$, providing new constraints on dark particle models.

Findings

No signal observed in the specified mass ranges.

Established upper limits on branching fractions at 90% confidence level.

Constrained parameter space for dark particles in the studied mass regions.

Abstract

We present the first search for the pair production of dark particles $X$ via $K_L^0\to XX$ with $X$ decaying into two photons using the data collected by the KOTO experiment. No signal was observed in the mass range of 40 - 110 MeV/c$^2$ and 210 - 240 MeV/c$^2$. This sets upper limits on the branching fractions as $\mathcal{B}(K_L^0 \to XX)$ $<$ (1-4) $\times$ 10$^{-7}$ and $\mathcal{B}(K_L^0 \to XX)$ $<$ (1-2) $\times$ 10$^{-6}$ at the 90% confidence level for the two mass regions, respectively.