A new search for the KL→π0νν¯ and KL→π0X0 decays

Abstract

We searched for the $CP$-violating rare decay of neutral kaon, $K_{L} \\to\n\\pi^0 \\nu \\overline{\\nu}$, in data from the first 100 hours of physics running\nin 2013 of the J-PARC KOTO experiment. One candidate event was observed while\n$0.34\\pm0.16$ background events were expected. We set an upper limit of\n$5.1\\times10^{-8}$ for the branching fraction at the 90\\% confidence level\n(C.L.). An upper limit of $3.7\\times10^{-8}$ at the 90\\% C.L. for the $K_{L}\n\\to \\pi^{0} X^{0}$decay was also set for the first time, where $X^{0}$ is an\ninvisible particle with a mass of 135 MeV/$c^{2}$.\n