A new search for the $K_L\to \ddot{I}{€}^0\hat{I}½\overline{\hat{I}}½$ and $K_L\to \ddot{I}{€}^0{X}^0$ decays

Abstract

We searched for the $CP$-violating rare decay of neutral kaon, $K_L\to \ddot{I}{€}^0\hat{I}½\overline{\hat{I}}½$, in data from the first 100 hours of physics running in 2013 of the J-PARC KOTO experiment. One candidate event was observed while $0.34\pm 0.16$ background events were expected. We set an upper limit of $5.1\times 10^{-8}$ for the branching fraction at the 90\% confidence level (C.L.). An upper limit of $3.7\times 10^{-8}$ at the 90\% C.L. for the $K_L\to \ddot{I}{€}^0{X}^0$decay was also set for the first time, where $X^0$ is an invisible particle with a mass of 135 MeV/$c^2$.

11 pages, 5 figures, to be published in PTEP