Peccei–Quinn theory promotes the physical QCD theta angle to a dynamical field through an anomalous global symmetry. Nonperturbative QCD drives its vacuum expectation value to a CP-conserving minimum.
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.
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The Peccei-Quinn theory is a theoretical framework proposed to address the strong CP (Charge-Parity) problem in quantum chromodynamics (QCD), which is the fundamental theory of the strong interaction between quarks and gluons. Formulated in 1977 by Roberto Peccei and Helen Quinn, the theory introduces a new class of particles called axions, which are proposed as a solution to this problem.