= Solution
An anomalous chiral redefinition moves phase between the bare QCD angle and the quark mass matrix, so the measurable parameter is
$$
\bar\theta=\theta_{\rm QCD}+\arg\det(M_uM_d).
$$
The neutron <electric dipole moment> requires $|\bar\theta|$ to be extremely small, although no generic Standard Model symmetry enforces this. This is the <Strong CP problem>.
In <Peccei–Quinn theory>, an anomalous spontaneously broken global symmetry replaces $\bar\theta$ by a dynamical <axion> field. Nonperturbative QCD generates an axion potential whose minimum lies at the CP-conserving value, dynamically relaxing the effective angle to zero.
Back to article page
