Axion can refer to different concepts depending on the context: 1. **Physics**: In particle physics, an axion is a hypothetical elementary particle that is proposed as a solution to the strong CP (Charge Parity) problem in quantum chromodynamics (QCD). It is a lightweight, neutral particle that could help explain why strong interactions do not seem to violate CP symmetry.
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The axion is the pseudo-Goldstone boson of spontaneously broken Peccei–Quinn symmetry. Its QCD-generated potential dynamically relaxes the physical theta angle toward zero.