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AxionThis article is about the hypothetical particle. Axion is also a genus of beetle of the ladybird family: see Axion (beetle)
The axion is a hypothetical exotic particle postulated by Peccei-Quinn theory to resolve the strong-CP problem in quantum chromodynamics (QCD). The naïve first principles formulation of QCD without axions predicts that some strong interactions will violate CP-symmetry. This is never observed in practice, and the axion was postulated to be a particle (specifically a pseudo-Goldstone boson) associated with a new broken symmetry of nature, whose conservation is constructed to exclude all CP-violating terms from QCD. Axions are predicted to have no electric charge, a very small mass in the range from 10-6 to 10-2 eV/c2, and very low interaction cross-sections for strong and weak forces. Hence they are nearly invisible to ordinary matter, and cannot be excluded on the basis of current measurements, though they have not been observed so far. The CAST experiment is currently underway to detect axions produced in the core of the Sun. Theory further suggest that axions were created abundantly during the big bang. Because of a unique coupling to the instanton field of the primordial universe (i.e., "misalignment mechanism"), an effective dynamical friction is created during the acquisition of mass following cosmic inflation. This robs all such primordial axions of their kinetic energy. Hence axion theories predict that the universe is filled with a very cold Bose-Einstein condensate of primordial axions. Depending on their mass, axions could plausibly explain the dark matter problem of cosmology. Observational studies to detect dark matter axions are underway, but they are not yet sufficiently sensitive to probe the mass regimes where axions would be expected to be found if they are the solution to the dark matter problem. Such studies have excluded the possibility of high mass axions. It should be noted that the existence of axions is also a necessary component of string theory. Links November 24th news article by PhysicsWeb.org CAST Experiment CAST at MPI/MPE
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