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

Area of Science:

• Particle Physics
• Cosmology
• Dark Matter Physics

Background:

• The Standard Model of particle physics has limitations, including the existence of dark matter.
• Dark photons are hypothetical particles that could mediate interactions between visible and dark sectors.
• Kaon decays offer a unique window to search for new physics beyond the Standard Model.

Purpose of the Study:

• To search for evidence of an invisible particle X, potentially a dark photon, in the decay of neutral kaons (K_{L}^{0}).
• To set upper limits on the branching ratio of the decay K_{L}^{0}→γX.
• To constrain the properties of dark photons, including their mass and coupling strengths.

Main Methods:

• Analysis of data from K_{L}^{0} decays searching for events with a single photon and missing energy.
• Statistical analysis to compare observed events with predicted background contributions.
• Calculation of upper limits on branching ratios and constraints on dark photon parameters.

Main Results:

• No significant evidence for the invisible particle X was observed.
• 13 candidate events were found, consistent with a predicted background of 12.66±4.42_{stat}±2.13_{syst} events.
• Upper limits were set on the branching ratio of K_{L}^{0}→γX for X masses between 0 and 425 MeV/c².

Conclusions:

• The search placed a stringent upper limit of 3.4×10^{-7} on the branching ratio for massless dark photons, improving previous indirect bounds by over three orders of magnitude.
• A new lower bound of 4.1×10^{6} TeV was established for the effective mass scale of flavor-changing dark photon couplings, a significant improvement over existing constraints.
• For massive dark photons, upper limits in the searched mass range varied from O(10^{-7}) to O(10^{-3}).