Axion
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The axion is a hypothetical exotic subatomic particle postulated by Peccei-Quinn theory to resolve the strong-CP problem in quantum chromodynamics (QCD).
The name was introduced by Frank Wilczek after a brand of detergent.
- This article is about the hypothetical particle. For genus of beetle of the ladybird family called axion, see Axion (beetle)
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Reasons for Prediction
The naïve first principles formulation of QCD (without axions) predicts that some strong interactions will violate CP-symmetry. Contrary to expectation, violation of CP by the strong force has never been observed. The axion was postulated to be the particle associated with a new broken symmetry of nature, whose conservation is constructed to exclude all CP-violating terms from QCD.
Axions can appear as effective degrees of freedom after integrating out charged chiral fields which undergo fermion condensation.
It should be noted that the existence of axions is also a necessary component of string theory.
Expected Properties
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. Because of their expected properties, if they do exist, axions must be nearly invisible to ordinary matter.
The postulated axions are pseudo-Goldstone bosons.
Although axions have not been observed so far, they cannot be disproved on the basis of current measurements. The CAST experiment is currently underway to detect axions produced in the core of the Sun. The finding in March 2006 of unexpected minuscule light polarization rotation in strong magnetic fields by the Italian PVLAS experiment may be the first experimental result suggesting the existence of axions.
In supersymmetric theories the axion has both a scalar and a fermionic superpartner. The fermionic superpartner of the axion is called the axino, the scalar superpartner is called the saxion.
Cosmologic Implications
Theory further suggests 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 regions 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.