Phenomenology of the companion-axion model: photon couplings

Understand

We study the phenomenology of the 'companion-axion model' consisting of two coupled QCD axions.

• The second axion is required to rescue the Peccei-Quinn solution to the strong-CP problem from the effects of colored gravitational instantons.
• We investigate here the combined phenomenology of axion-axion and axion-photon interactions, recasting present and future single-axion bounds onto the companion-axion parameter space.
• Most remarkably, we predict that future axion searches with haloscopes and helioscopes may well discover two QCD axions, perhaps even within the same experiment.