Search for $K_L\to \ddot{I}{€}^0\hat{I}½\overline{\hat{I}}½$ and $K_L\to \ddot{I}{€}^0{X}^0$ Decays at the J-PARC KOTO Experiment

Abstract

A search for the rare decay $K_L\to \ddot{I}{€}^0\hat{I}½\overline{\hat{I}}½$ was performed. With the data collected in 2015, corresponding to $2.2\times 10^{19}$ protons on target, a single event sensitivity of $(1.30\pm 0.01_{\mathrm{stat}}\pm 0.14_{\mathrm{syst}})\times 10^{-9}$ was achieved and no candidate events were observed. We set an upper limit of $3.0\times 10^{-9}$ for the branching fraction of $K_L\to \ddot{I}{€}^0\hat{I}½\overline{\hat{I}}½$ at the 90% confidence level (C.L.), which improved the previous limit by almost an order of magnitude. An upper limit for $K_L\to \ddot{I}{€}^0{X}^0$ was also set as $2.4\times 10^{-9}$ at the 90% C.L., where $X^0$ is an invisible boson with a mass of $135\mathrm{MeV}/c^2$.

6 pages, 4 figures; published version, no change in the results. Fig. 1, Fig. 2, and Fig. 3 were revised, and Fig. 4 presenting the $K_L\to \ddot{I}{€}^0{X}^0$ limit as a function of the $X^0$ mass was added