A new search for the $K_{L} \to \pi^0 \nu \overline{\nu}$ and $K_{L} \to   \pi^{0} X^{0}$ decays
  We searched for the $CP$-violating rare decay of neutral kaon, $K_{L} \to \pi^0 \nu \overline{\nu}$, in data from the first 100 hours of physics running in 2013 of the J-PARC KOTO experiment. One candidate event was observed while $0.34\pm0.16$ background events were expected. We set an upper limit of $5.1\times10^{-8}$ for the branching fraction at the 90\% confidence level (C.L.). An upper limit of $3.7\times10^{-8}$ at the 90\% C.L. for the $K_{L} \to \pi^{0} X^{0}$decay was also set for the first time, where $X^{0}$ is an invisible particle with a mass of 135 MeV/$c^{2}$. 
J. K. Ahn, K. Y. Baek, S. Banno, B. Beckford, B. Brubaker, T. Cai, M.   Campbell, C. Carruth, S. H. Chen, S. Chu, J. Comfort...
hep-ex