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

We searched for the CP-violating rare decay of the neutral kaon, KL→π0νν¯, 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±0.16 background events were expected. We set an upper limit of 5.1×10−8 for the branching fraction at the 90% confidence level (C.L.). An upper limit of 3.7×10−8 at the 90% C.L. for the KL→π0X0 decay was also set for the first time, where X0 is an invisible particle with a mass of 135 MeV/c2.