Phenomenology of the companion-axion model: photon couplings
  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.
