axionphoton

The axion–photon interaction refers to the coupling between axions, hypothetical pseudoscalar particles proposed to solve the strong CP problem in quantum chromodynamics, and photons. In particle theory, this interaction is often described by a term in the Lagrangian that, to leading order, allows axions to convert into photons and photons into axions in the presence of electric or magnetic fields. The coupling strength is denoted g_{aγγ} and the interaction leads to observable effects such as a modulated photon field in strong magnetic environments.

In practical terms, a background magnetic field enables axion–photon conversion via the Primakoff effect. This means that in astrophysical contexts—such as stars or interstellar magnetic fields—or in laboratory experiments that use strong magnets, axions can convert to photons with a characteristic energy corresponding to the axion mass, and photons can convert to axions and escape detectors or walls.

The axion–photon coupling has important implications for both cosmology and astrophysics. If axions make up dark matter, they would induce a narrow spectral line in the radio or microwave range. Axion–photon conversion can also affect stellar cooling and photon propagation in magnetic fields, providing constraints from observations of stars and supernovae.

Experimental searches employ several approaches. Light shining through walls experiments test axion–photon conversion in strong magnets. Solar axion searches use helioscopes such as CAST and the proposed IAXO. Dark-matter axion searches, or haloscopes, include experiments like ADMX and HAYSTAC, as well as dielectric haloscope concepts such as MADMAX. To date, no definitive detection has been reported, but the parameter space is actively constrained and explored.