Strong CP problem

The strong CP problem is the unexplained smallness of quantum chromodynamics’ effective angle θ, which would otherwise produce observable charge-parity violation, including a neutron electric dipole moment.

QCD vacuum angle: The dimensionless parameter multiplying the CP-violating topological term in the QCD Lagrangian. Its unexplained near-zero value is the central quantity in the strong CP problem.
Axion: A hypothetical light pseudoscalar particle arising from the Peccei–Quinn solution to the strong CP problem. It dynamically drives the effective QCD angle toward zero.
Massless up quark: A proposed solution in which the up-quark mass is exactly zero, making the QCD angle removable by a chiral rotation. It would eliminate the physical angle without an axion, but is disfavored by hadron data.
Quantum chromodynamics: The quantum field theory describing quarks, gluons, and the strong interaction. Its topology and quark masses produce the terms behind the puzzle.
Axion–photon coupling: The interaction strength connecting axions to two photons, governing many laboratory and astrophysical searches. Its value determines whether axions can be detected in proposed experiments.
Topological charge: An integer-valued quantity characterizing the winding of a gauge field configuration. QCD’s topologically distinct field configurations make the vacuum angle physically consequential.
Peccei–Quinn mechanism: A proposed global symmetry mechanism that makes the QCD vacuum angle a dynamical field. It converts the unexplained parameter into a field whose equilibrium value cancels CP violation.
Nelson–Barr mechanism: A class of models that permits spontaneous CP violation while keeping the QCD angle zero at tree level. It solves strong CP through additional fields and symmetries rather than a dynamical axion.
Charge-parity symmetry: A symmetry combining charge conjugation, which exchanges particles and antiparticles, with spatial parity inversion. The strong CP problem asks why QCD appears to preserve this combined symmetry so precisely.
Axion mass: The mass of the hypothetical axion, which depends on the scale of Peccei–Quinn symmetry breaking. The strong CP solution permits a range of masses that experiments are still exploring.

Linked from 10 pages