Search for Dark Particles in K_{L}^{0}→γX at the KOTO Experiment

We report a search for an invisible particle X in the decay K_{L}^{0}→γX (X→invisible), where X can be interpreted as a massless or massive dark photon. No evidence for X was found, based on 13 candidate events consistent with a predicted background of 12.66±4.42_{stat}±2.13_{syst} events. Upper limits on the branching ratio of K_{L}^{0}→γX were set for the X mass range 0≤m_{X}≤425  MeV/c^{2}. For massless X, the upper limit was 3.4×10^{-7} at the 90% confidence level, improving the previous indirect bound by over three orders of magnitude. This result yielded a new lower bound of 4.1×10^{6}  TeV on the probed effective mass scale governing flavor-changing dark-photon couplings, representing approximately two orders of magnitude improvement on existing constraints. For massive X, the upper limits in the searched mass region ranged from O(10^{-7}) to O(10^{-3}).