Probing a dark photon using rare leptonic kaon and pion decays

Rare leptonic kaon and pion decays K+(π+)→μ+νμe+e− can be used to probe a dark photon of mass O(10)MeV, with the background coming from the mediation of a virtual photon. This is most relevant for the 16.7-MeV dark photon proposed to explain a 6.8σ anomaly recently observed in 8Be transitions by the Atomki Collaboration. We evaluate the reach of future experiments for the dark photon with vectorial couplings to the standard model fermions except for the neutrinos, and show that a great portion of the preferred 16.7-MeV dark photon parameter space can be decisively probed. We also show the use of angular distributions to further distinguish the signal from the background.