Search for KL →π0ν ν and KL →π0X0 Decays at the J-PARC KOTO Experiment

Abstract

A search for the rare decay KL→π0νν was performed. With the data collected in 2015, corresponding to 2.2×1019 protons on target, a single event sensitivity of (1.30±0.01stat±0.14syst)×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 KL→π0νν at the 90% confidence level (C.L.), which improved the previous limit by almost an order of magnitude. An upper limit for KL→π0X0 was also set as 2.4×10-9 at the 90% C.L., where X0 is an invisible boson with a mass of 135 MeV/c2.