π-axion interactions and their parameter constraints

Axions and axion-like particles are leading dark matter candidates which appear in many extensions of the Standard Model. In their paper, "The π-axion and the π-axiverse of dark QCD," Alexander et al. propose a dark copy of the standard model wherein the photon is massive and the 6 quark flavors are light. In their model, dark matter can take the form of dark pions in the axion mass range. These 35 'π-axions' are grouped together into neutral, complex neutral, and complex charged states, and have parity-odd and parity-even interactions with dark and visible photons. We present constraints on the couplings of these π-axions to photons. We use Tilburg et al.’s limits on couplings between ultralight scalar dark matter and photons to constrain the parity-even π-axion-photon coupling. We find a constraint that depends on both the masses and the relative densities of the π-axions involved, with a lower bound of roughly 3.89 × 10⁴ GeV. These constraints offer new insights into the behavior of π-axions and opportunities to distinguish them from other dark matter candidates.