In my last dark matter post, I described WIMPs. While WIMPs are a very attractive dark matter candidate, they are by no means the only one. Axions are a much more difficult particle to describe than WIMPs. As an experimental particle physicist, even I don’t understand a lot about them (though I admittedly have never studies them in detail). As usual, you can look at the PDG axion review for more detailed info (that’s where a lot of the information I’m using here is from).
Axions were first proposed by Peccei and Quinn to solve a very different problem in particle physics: the strong CP problem. It turns out that in a generic theory of quantum chromodynamics (QCD), we would expect there to a be a CP-violating term ΘGμνGμν where Θ determines the amount of CP violation. CP violation in strong interactions can be found by looking for things like the existence of the neutron electric dipole moment. If CP is preserved, this must be exactly 0. Experimental bounds have found no evidence for the existence of the neutron EDM, so the parameter Θ must be incredibly small. Such a coincidence would seem very strange to physicists, so this suggests that maybe there is something that forces this to happen. The axion is the particle arising from a scalar/pseudoscalar field added (using a new U(1) symmetry) to force QCD to preserve CP symmetry.
The axion is expected to interact with photons through an interaction term that looks like gφFμνFμν where the coupling constant g is related to the axion mass. This in particular, allows for the axion to be converted into a photon in the presence of a strong electromagnetic field. Most experimental searches try to find axions using strong magnetic fields in resonating cavities.
In the early universe, while axions can be formed through the usual thermal processes, these will lead to hot (relativistic) axions that probably won’t be a suitable dark matter candidate. However, a condensate of axions can be created during spontaneous symmetry breaking, generating a large number of non-relativistic axions. It turns out that for masses around 10 μeV the correct relic density is achieved. So, dark matter axions will have incredibly tiny masses, likely even less than neutrinos and, as for WIMPs, could be related to the solution for several open questions about the Standard Model.