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

Abstract

We report a search for an invisible particle $X$ in the decay $K^0_L\rightarrow γX$ ($X \to \text{invisible}$), 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 $K^0_L\rightarrow γX$ 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 $O(10^{-3})$.

Figures (5)

• Figure 1: Cross-sectional view of the KOTO detector, with the beam entering from the left. The detector components with their names underlined represent charged-particle veto counters. The other components, except for CSI, serve as photon veto counters.
• Figure 2: Distributions of $E_{\gamma}$ and $H_{XY}$ after requiring no in-time signal in veto counters. Black dots represent the data, while the hollow histograms show the contributions from each background source indicated in the figure. "Other kaon decays" include all $K_L^0$ backgrounds summarized in \ref{['tab:BGSummary']}, excluding $K^0_L\rightarrow \gamma \gamma$. The neutron data corresponds to samples collected in dedicated neutron runs. The shaded histogram represents the sum of all backgrounds.
• Figure 3: Distributions of $\Delta T$. The hollow histogram shows the photon sample from $K^0_L\rightarrow\pi^0\pi^0\pi^0$ data, while the shaded histogram represents the neutron sample from dedicated neutron runs.
• Figure 4: Distribution of $E_\gamma$ versus $H_{XY}$ after all selection criteria for $K^0_L\rightarrow \gamma X$. The red rectangle indicates the signal region. Black dots are the observed events, and the shaded contour shows the $K^0_L\rightarrow \gamma X$ MC distribution for massless $X$. Numbers in black (red) represent the observed (expected) event counts.
• Figure 5: Upper limit at the 90% C.L. on the branching fraction of the $K^0_L\rightarrow \gamma X$ decay as a function of the $X$ mass.