Enhanced axion–photon coupling in GUT with hidden photon

We show that the axion coupling to photons can be enhanced in simple models with a single Peccei–Quinn field, if the gauge coupling unification is realized by a large kinetic mixing χ=O(0.1) between hypercharge and unbroken hidden U(1)H. The key observation is that the U(1)H gauge coupling should be rather strong to induce such large kinetic mixing, leading to enhanced contributions of hidden matter fields to the electromagnetic anomaly. We find that the axion–photon coupling is enhanced by about a factor of 10–100 with respect to the GUT-axion models with E/N=8/3.