# Hunting down massless dark photons in kaon physics

## TL;DR

This paper explores the potential for massless dark photons to cause rare kaon decays via flavor-changing magnetic-dipole couplings, suggesting detectable signals in experiments like NA62.

## Contribution

It introduces a novel analysis of massless dark photon effects in kaon physics, focusing on flavor-changing couplings and their experimental signatures.

## Key findings

- Branching ratios up to 10^{-7} are possible.
- Massless dark photons could produce detectable signals in rare kaon decay experiments.
- The study provides a framework for future experimental searches.

## 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 $K^+\rightarrow \pi^+\pi^0 \bar \gamma$ 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 $K^+\rightarrow \pi^+\pi^0 \bar \gamma$ 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.