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?