Study of the $K_L\to \ddot{I}{€}^0\hat{I}½\overline{\hat{I}}½$ Decay at the J-PARC KOTO Experiment

Abstract

The rare decay $K_L\to \ddot{I}{€}^0\hat{I}½\overline{\hat{I}}½$ was studied with the dataset taken at the J-PARC KOTO experiment in 2016, 2017, and 2018. With a single event sensitivity of $(7.20\pm 0.05_{\mathrm{stat}}\pm 0.66_{\mathrm{syst}})\times 10^{-10}$, three candidate events were observed in the signal region. After unveiling them, contaminations from $K^{\pm }$ and scattered $K_L$ decays were studied, and the total number of background events was estimated to be $1.22\pm 0.26$. We conclude that the number of observed events is statistically consistent with the background expectation. For this dataset, we set an upper limit of $4.9\times 10^{-9}$ on the branching fraction of $K_L\to \ddot{I}{€}^0\hat{I}½\overline{\hat{I}}½$ at the 90% confidence level.

6 pages, 5 figures; published version, no change in the results. Fig. 2 and Fig. 4 were revised. In Fig. 5, the plot of the $P_t$ versus $Z_{\mathrm{vtx}}$ of the beam-halo $K_L\to 2{\hat{I}}^3$ background events was added as Fig. 5 (b)