The KOTO experiment at J-PARC searches for the CP-violating rare decay KL->pi0 nu nubar. The branching ratio of this decay is predicted to be 3e-11 in the standard model (SM). Because of the small theoretical uncertainty in the prediction, the decay is considered to be a good testing ground to explore new physics beyond the SM.

KOTO presented preliminary findings on the search in the analysis of the data taken from 2016 to 2018 at the KAON conference in 2019. At the time, existence of four candidate events in the signal region with a small background expectation was reported, with a single event sensitivity of 7e-10. After unveiling them, we further studied possible background sources and found that backgrounds cause by charged kaon contamination in the neutral beam and scattered KL decays could be larger than expectation. We confirmed their contributions by using special data taken in 2020 and concluded that the number of the observed events is statistically consistent with the background expectation. Details of the analysis will be explained in this seminar.

In addition we will discuss prospects of the KOTO experiment, including sensitivity projection in near future and next generation experiment, KOTO Step-2, for further sensitivity that aims to observe O(100) events.

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