axions

Axions are hypothetical elementary particles, light pseudoscalar bosons proposed to resolve the strong CP problem in quantum chromodynamics by introducing a global Peccei-Quinn symmetry that is spontaneously broken. They arise as a consequence of the dynamic relaxation of the CP-violating parameter in QCD to a CP-conserving value.

In most models axions are extremely light and interact only very weakly with ordinary matter, making them viable dark matter candidates. Their mass is inversely related to the Peccei-Quinn breaking scale f_a; approximately m_a ≈ 6 × 10^-6 eV when f_a = 10^12 GeV, with theoretical ranges spanning microelectronvolts to millielectronvolts depending on the model. The coupling to photons and other Standard Model particles is also suppressed by f_a and varies by model, with the axion-photon coupling often emphasized in experimental searches.

Cosmologically, axions can be produced non-thermally in the early universe through the misalignment mechanism and, in certain scenarios, via decay of cosmic strings or domain walls. This can yield a cold dark matter population. Astrophysical constraints arise from axion emission in stars and supernovae, which limit their couplings, while cosmological considerations constrain their abundance and velocity distribution.

Experimental searches for axions fall into several programs. Haloscopes, such as the ADMX experiment, aim to detect dark matter axions converting to photons in resonant microwave cavities within strong magnetic fields. Helioscopes, like CAST and the planned IAXO, look for axions produced in the Sun. Other approaches include dielectric haloscopes and light-shining-through-walls experiments. To date, no definitive axion signal has been observed. Axion-like particles (ALPs) extend the concept to a broader class of light pseudoscalars with similar couplings but not necessarily tied to the strong CP problem.