Axion
| Interactions | Gravity, electromagnetic |
|---|---|
| Status | Hypothetical |
| Symbol | A0 |
| Theorized | 1977, Peccei and Quinn |
| Mass | 10−5 to 10−3 eV/c2 |
| Electric charge | 0 |
| Spin | 0 |
The axion is a hypothetical elementary particle postulated by the Peccei–Quinn theory in 1977 to resolve the strong CP problem in quantum chromodynamics (QCD). If axions exist and have low mass within a specific range, they are of interest as a possible component of cold dark matter.
As shown by Gerardus 't Hooft, strong interactions of the standard model, QCD, possess a non-trivial vacuum structure that in principle permits violation of the combined symmetries of charge conjugation and parity, collectively known as CP. Together with effects generated by weak interactions, the effective periodic strong CP-violating term, Θ, appears as a Standard Model input – its value is not predicted by the theory, but must be measured. However, large CP-violating interactions originating from QCD would induce a large electric dipole moment (EDM) for the neutron. Experimental constraints on the currently unobserved EDM implies CP violation from QCD must be extremely tiny and thus Θ must itself be extremely small. Since a priori Θ could have any value between 0 and 2π, this presents a naturalness problem for the standard model. Why should this parameter find itself so close to 0? (Or, why should QCD find itself CP-preserving?) This question constitutes what is known as the strong CP problem.
One simple solution exists: if at least one of the quarks of the standard model is massless, Θ becomes unobservable. However, empirical evidence strongly suggests that none of the quarks are massless.
In 1977, Roberto Peccei and Helen Quinn postulated a more elegant solution to the strong CP problem, the Peccei–Quinn mechanism. The idea is to effectively promote Θ to a field. This is accomplished by adding a new global symmetry (called a Peccei–Quinn symmetry) that becomes spontaneously broken. This results in a new particle, as shown by Frank Wilczek and Steven Weinberg, that fills the role of Θ, naturally relaxing the CP-violation parameter to zero. This hypothesized new particle is called the axion. The original Weinberg–Wilczek axion was ruled out. Current literature discusses the mechanism as the "invisible axion", which has two forms: KSVZ (Kim-Shifman-Vainshtein-Zakharov) and DFSZ (Dine-Fischler-Srednicki-Zhitnitsky).
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