Abstract
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.
Highlights
The axion, a, is a pseudo-Nambu-Goldstone boson associated with spontaneous breakdown of a global U(1)PQ symmetry in the Peccei-Quinn (PQ) mechanism [1,2,3,4]
In this paper we study the axion coupling to photons in a grand unified theories (GUTs) scenario where a massless hidden photon has a large kinetic mixing with hypercharge
We have shown that the axion coupling to photons can be enhanced in a simple model with a single PecceiQuinn breaking field, if the gauge coupling unification is realized by a large kinetic mixing between hypercharge and unbroken hidden U(1)H
Summary
The axion, a, is a pseudo-Nambu-Goldstone boson associated with spontaneous breakdown of a global U(1)PQ symmetry in the Peccei-Quinn (PQ) mechanism [1,2,3,4]. One can consider a clockwork (or aligned) QCD axion model [11,12,13], which include multiple PQ fields with a clockwork structure [14,15,16,17,18] In this case, the axion can have a coupling to photons [19] or hidden photons [12], which is exponentially larger than gluons. The large kinetic mixing and strong U(1)H gauge coupling enhance the electromagnetic anomaly, and the axion coupling to photons can be enhanced even in a simple model with a single PQ field. Such enhancement is advantageous for the axion search experiments utilizing the the axion photon coupling. The last section is devoted to discussion and conclusions
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