In particle physics, the Peccei–Quinn theory is a well-known proposal for the resolution of the strong CP problem. It was formulated by Roberto Peccei and Helen Quinn. The theory proposes that the QCD Lagrangian be extended with a CP-violating term known as the θ term. Because experiments have never measured a value for θ, its value must be small if it exists.
Peccei–Quinn theory predicts that the small θ parameter is explained by a dynamic field, rather than a constant value. Because particles arise within quantum fields, Peccei–Quinn theory predicts the existence of a new particle, the axion. The potential which this field carries causes it to have a value which naturally cancels, making the θ parameter uneventfully zero.
Peccei–Quinn symmetry presents θ as a functional component—a global U(1) symmetry under which a complex scalar field is charged. This symmetry is spontaneously broken by the vacuum expectation value obtained by this scalar field, and the axion is the massless Goldstone boson of this broken symmetry. If it is a gauge symmetry then the axion is superseded by the gauge boson, meaning that the gauge boson becomes massive and the axion is no longer observable (see Higgs mechanism). This is phenomenologically desirable because it leaves no massless particles, which are indeed not seen experimentally.
This Peccei–Quinn symmetry is inexact because it is anomalously broken by QCD instantons. If there is a compensatory term canceling the QCD anomaly breaking term, the axion becomes an exactly massless Goldstone boson and θ is no longer fixed. The effective potential of the axion is the summed potential above the QCD scale; with the potential term induced by nonperturbative QCD effects. If the axion is fundamental, or emerges at a scale far higher than the QCD scale, then the dimension 5 axion coupling term