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A search for the rare decay K L → π 0 νν was performed. With the data collected in 2015, corresponding to 2.2 × 10 19 protons on target, a single event sensitivity of ð1.30 AE 0.01 stat AE 0.14 syst Þ × 10 −9 was achieved and no candidate events were observed. We set an upper limit of 3.0 × 10 −9 for the branching fraction of K L → π 0 νν at the 90% confidence level (C.L.), which improved the previous limit by almost an order of magnitude. An upper limit for K L → π 0 X 0 was also set as 2.4 × 10 −9 at the 90% C.L., where X 0 is an invisible boson with a mass of 135 MeV=c 2 .

Three events for the decay K+-->pi+ nunu have been observed in the pion momentum region below the K+-->pi+pi0 peak, 140 < Ppi < 199 MeV/c, with an estimated background of 0.93+/-0.17(stat.) -0.24+0.32(syst.) events. Combining this observation with previously reported results yields a branching ratio of B(K+-->pi+ nunu) = (1.73(-1.05)+1.15) x 10(-10) consistent with the standard model prediction.