Search for Dark Particles in KL0 →γX at the KOTO Experiment

We report a search for an invisible particle X in the decay KL0→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±4.42stat±2.13syst events. Upper limits on the branching ratio of KL0→γX were set for the X mass range 0≤mX≤425 MeV/c2. 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 4.1×106 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).