Hunting down massless dark photons in kaon physics

Abstract

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 $K^{+}\to {\pi }^{+}{\pi }^0\bar{\gamma }$ with a simplified-model approach, assuming the chiral quark model to evaluate the hadronic matrix element. Possible effects in the $K^0$-${\bar{K}}^0$ 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 $K^{+}\to {\pi }^{+}{\pi }^0\bar{\gamma }$ could be of interest for experiments dedicated to rare $K^{+}$ decays like NA62 at CERN, where $\bar{\gamma }$ can be detected as a massless invisible system.

Comments: same as the published PRL version, 5 pages, 3 figures, a few comments and new references added