Search For K-L -> Pi(0)V(V)Over-Bar And K-L -> Pi X-0(0) Decays At The J-Parc Koto Experiment

A search for the rare decay K-L -> pi(0)v (v) over bar was performed. With the data collected in 2015, corresponding to 2.2 x 10(19) protons on target, a single event sensitivity of (1.30 +/- 0.01(stat) +/- 0.14(syst)) x10(-9) was achieved and no candidate events were observed. We set an upper limit of 3.0 x 10(-9) for the branching fraction of K-L -> pi(0)v (v) over bar at the 90% confidence level (C.L.), which improved the previous limit by almost an order of magnitude. An upper limit for K-L -> pi X-0(0) was also set as 2.4 x 10(-9) at the 90% C.L., where X-0 is an invisible boson with a mass of 135 MeV/c(2).