Axions are hypothetical particles that arise in some extensions of the Standard Model of particle physics. They are predicted to be very light, and to interact only very weakly with ordinary matter. Nevertheless, they could play an important role in astrophysics and cosmology. For example, axions could make up the dark matter that is thought to permeate the Universe.
The name “axion” comes from a particular kind of symmetry that is predicted to exist in certain theories beyond the Standard Model. This symmetry, called Peccei-Quinn symmetry, was proposed in 1977 by Roberto Peccei and Helen Quinn as a way to solve the so-called strong CP problem. The strong CP problem is a puzzle that arises in quantum chromodynamics (QCD), the theory of the strong nuclear force. According to QCD, there should be a contribution to the electric dipole moment of particles like the neutron which violates both parity (P) and charge conjugation (C). However, experiments have shown that this contribution must be incredibly small – much smaller than what QCD predicts. The Peccei-Quinn symmetry provides a natural explanation for why this should be so: it implies that there must be a new particle – the axion – whose presence cancels out the unwanted contribution to the electric dipole moment.
The properties of axions depend on their mass, which is unknown but constrained by experimental searches and theoretical considerations. If axions are indeed responsible for dark matter, they are expected to have a mass of around 10^(-5) eV/c^2 – many orders of magnitude lighter than any other known particle! This makes them extremely difficult to detect directly. However, there are ongoing efforts to search for axions using sensitive laboratory detectors; if successful, these searches could uncover this elusive particle and shed new light on our understanding of fundamental physics.