# Axion coupling to photons

I dont understand the answer to this question: axion couplings

There are experiments trying to measure how light is shining through a wall, using the coupling of the strong cp fixing axion to the sm photon.

But is this coupling of the axion directly to the photon (as I saw in many related Feynman diagrams) or via a fermion loop as suggested by the answer in the above stated link?

So in other words is there a $a F^{\mu\nu} F_{\mu\nu}$ term in the suggested lagrangian where F is the QED F and a the axion field?

• ? You are asking the same question, and failing to appreciate its virtually unimpeachable answer. In terms of fundamental fields, the axion coupling in that effective term comes out of triangle diagrams with intermediate quarks. The effective action term you wrote is what results, of course it exists, and is what is being applied to the Feynman diagrams you may have seen. – Cosmas Zachos Mar 29 '17 at 16:11
• Ok fine, so the actual coupling is via the fermion loop and feynman diagrams leaving out this loop use an effective coupling? – Mr Puh Mar 30 '17 at 9:13