Search for the pair production of dark particles in KL0 decays at KOTO

Abstract We present the first search for the pair production of dark particles X in K L 0 decays assuming that X decays into two photons promptly. No signal was observed in the mass regions of 40 − 110 MeV/c2 and 210 − 240 MeV/c2. This leads to B ( K L 0 → X X ) < ( 1 − 4 ) × 10 − 7 and B ( K L 0 → X X ) < ( 1 − 2 ) × 10 − 6 at the 90% confidence level for the two mass regions, respectively.