PecceiQuinn

The Peccei–Quinn mechanism is a proposed solution to the strong CP problem in quantum chromodynamics. While QCD allows a CP-violating theta term, experimental limits on the neutron electric dipole moment imply that the effective theta angle must be exceedingly small. Peccei and Quinn introduced a new global U(1) symmetry, spontaneously broken at a high energy scale, to address this tension.

When the U(1)PQ symmetry is broken, a pseudo-Goldstone boson called the axion emerges. The axion mass arises from nonperturbative QCD effects that explicitly break the symmetry, giving the axion a small mass. The dynamical evolution of the axion field drives the effective theta angle toward zero, dynamically solving the strong CP problem. The axion also couples to gluons and photons, with couplings suppressed by the symmetry-breaking scale f_a.

Two well-known implementations are KSVZ and DFSZ. In KSVZ models, new heavy quarks carry PQ charge while standard model fields may be neutral; in DFSZ models, standard model fermions carry PQ charge with additional scalar fields. The axion’s mass and couplings are inversely related to f_a, making the phenomenology model-dependent but generally very weakly interacting.

Axions are also considered a candidate for dark matter, produced in the early universe via misalignment and other mechanisms. Experiments such as microwave cavity searches and helioscopes probe the axion parameter space, while astrophysical observations place lower bounds on f_a (and upper bounds on the axion mass). The Peccei–Quinn framework remains a leading focus in particle physics and cosmology.