Axion

The axion is a hypothetical elementary particle proposed to solve the strong CP problem in quantum chromodynamics. It arises from the Peccei–Quinn mechanism, in which a new global U(1) symmetry is spontaneously broken. This breaking yields a light pseudoscalar particle, the axion, whose mass is generated by nonperturbative QCD effects and is extremely small compared with most standard-model particles.

The axion’s properties are largely determined by the scale f_a at which the Peccei–Quinn symmetry is broken. The axion mass m_a is inversely related to f_a, and the axion couples very weakly to ordinary matter and radiation. Different theoretical realizations—such as KSVZ (hadronic) and DFSZ models—specify how the axion interacts with Standard Model particles, in particular with photons via a coupling that enables conversion to photons in magnetic fields.

Axions can be produced in the early universe through mechanisms such as vacuum realignment, decay of topological defects, and thermal processes. If they exist and have the right abundance, axions would behave as cold dark matter today, moving non-relativistically in galaxies. They can convert into photons in the presence of strong magnetic fields (the Primakoff effect), a property exploited by various experimental searches.

Experimental efforts search for axions in two main ways: haloscopes look for dark-matter axions in the Galactic halo by detecting axion-to-photon conversion in resonant cavities within strong magnets; helioscopes search for axions produced in the Sun. No confirmed detection has yet been reported, but experiments such as ADMX and CAST have set limits on the axion-photon coupling over portions of parameter space. Ongoing and future experiments aim to cover broader ranges of mass and coupling.