Search for the KL →π0ν ν ¯ Decay at the J-PARC KOTO Experiment

We performed a search for the KL→π0νν¯ decay using the data taken in 2021 at the J-PARC KOTO experiment. With newly installed counters and new analysis method, the expected background was suppressed to 0.252±0.055stat -0.067syst+0.052. With a single event sensitivity of (9.33±0.06stat±0.84syst)×10-10, no events were observed in the signal region. An upper limit on the branching fraction for the decay was set to be 2.2×10-9 at the 90% confidence level (C.L.), which improved the previous upper limit from KOTO by a factor of 1.4. With the same data, a search for KL→π0X0 was also performed, where X0 is an invisible boson with a mass ranging from 1 to 260 MeV/c2. For X0 with a mass of 135 MeV/c2, an upper limit on the branching fraction of KL→π0X0 was set to be 1.6×10-9 at the 90% C.L.