If dark photons are massless, they couple to standard-model particles only via higher dimensional operators, while the kinetic mixing with photons, which motivates most of the current experimental searches, is absent. We consider the effect of possible flavor-changing magnetic-dipole couplings of massless dark photons in kaon physics. In particular, we study the branching ratio for the process K+→π+π0γ ̄ with a simplified-model approach, assuming the chiral quark model to evaluate the hadronic matrix element. Possible effects in the K0-K ̄0 mixing are taken into account. We find that branching ratios up to O(10−7) are allowed---depending on the dark-sector masses and couplings. Such large branching ratios for K+→π+π0γ ̄ could be of interest for experiments dedicated to rare K+ decays like NA62 at CERN, where γ ̄ can be detected as a massless invisible system.
