RESUMEN
We present the first search for the pair production of dark particles X via K_{L}^{0}âXX with X decaying into two photons using the data collected by the KOTO experiment. No signal was observed in the mass range of 40-110 MeV/c^{2} and 210-240 MeV/c^{2}. This sets upper limits on the branching fractions as B(K_{L}^{0}âXX)<(1-4)×10^{-7} and B(K_{L}^{0}âXX)<(1-2)×10^{-6} at the 90% confidence level for the two mass regions, respectively.
RESUMEN
The rare decay K_{L}âπ^{0}νν[over ¯] was studied with the dataset taken at the J-PARC KOTO experiment in 2016, 2017, and 2018. With a single event sensitivity of (7.20±0.05_{stat}±0.66_{syst})×10^{-10}, three candidate events were observed in the signal region. After unveiling them, contaminations from K^{±} and scattered K_{L} decays were studied, and the total number of background events was estimated to be 1.22±0.26. We conclude that the number of observed events is statistically consistent with the background expectation. For this dataset, we set an upper limit of 4.9×10^{-9} on the branching fraction of K_{L}âπ^{0}νν[over ¯] at the 90% confidence level.
