The Search for $K_L \rightarrow π^0π^0γγ$ and $K_L\rightarrow π^0π^0X$ where $X\rightarrow 2γ$ at the KOTO Experiment

Abstract

We performed searches for $K_L\rightarrow π^0π^0X$ where $X$ may be an axion-like particle which promptly decays to two photons, and the first search for $K_L \rightarrow π^0π^0γγ$ 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($K_L\rightarrow π^0π^0X$) $< (1\unicode{x2013}20) \times 10^{-7}$ at the 95% confidence level (C.L.). No events were observed for the analysis of $K_L \rightarrow π^0π^0γγ$, setting an upper limit on the branching ratio, BR($K_L \rightarrow π^0π^0γγ$) $< 1.69 \times 10^{-6}$ at the 95% C.L.

Figures (5)

• Figure 1: Cross-sectional side view of KOTO detector. A detector name with an underline implies it is a charged particle veto counter. All others, except for the CSI calorimeter, are photon veto counters.
• Figure 2: Invariant mass, $M_{\gamma_5\gamma_6}$, distribution in data, overlaid with the $K_L \rightarrow 3\pi^0$ background simulation normalized to the branching ratio. The signal simulation is weighted according to an assumed $\text{BR}(K_L \rightarrow \pi^0 \pi^0 X) = 1 \times 10^{-7}$ and $\text{BR}(K_L \rightarrow \pi^0 \pi^0 \gamma \gamma ) = 8.4\times 10^{-8}$funck_kambor.
• Figure 3: $K_L \rightarrow 3\pi^0$ data (black) and Monte Carlo (blue) comparison after applying the inverse selection and scale factor. The numbers represent the number of data and Monte Carlo events observed in the regions encapsulated by the black (red) lines.
• Figure 4: SES as a function of $X$ mass. The horizontal bar represents the SES of the $K_L \rightarrow \pi^0 \pi^0 \gamma \gamma$ decay. The error bars represent the quadratic sum of the statistical and systematic uncertainty which is propagated from the $K_L$ yield and $A_{sig}$.
• Figure 5: 95% C.L. upper limits. The width of each discrete line shows the ROI for the respective analysis. The points correspond to the three unique observed events and their invariant mass.