Imperfect Axion Precludes the Domain Wall Problem.

Area of Science:

• Particle Physics
• Cosmology
• Astroparticle Physics

Background:

• The strong CP problem in Quantum Chromodynamics (QCD) is typically addressed by the Peccei-Quinn mechanism, which introduces an axion.
• Standard axion models with discrete symmetries can lead to a cosmological domain wall problem if the axion field is not stabilized.
• The exact pseudoscalar nature of the QCD axion is not strictly required for solving the strong CP problem.

Purpose of the Study:

• To propose a dynamical solution to the axion domain wall problem.
• To investigate the cosmological implications of imperfect QCD axions with linear couplings to Standard Model particles.
• To establish constraints on axion properties based on proposed mechanisms and existing experimental limits.

Main Methods:

• Introduction of effective operators where the Peccei-Quinn field linearly couples to Standard Model particles.
• Analysis of the generation of a thermal potential that drives the axion field to a universal value at high temperatures.
• Combination of theoretical model predictions with existing experimental constraints, such as electric dipole moments and fifth force searches.

Main Results:

• A dynamical mechanism is proposed that prevents the formation of domain walls by stabilizing the axion field before the QCD phase transition.
• The proposed interactions generate a thermal potential, ensuring a homogeneous axion field value across the universe.
• A lower bound on the axion mass is derived to be approximately 10^{-5} eV when combined with current experimental constraints.

Conclusions:

• Imperfect QCD axions with specific couplings offer a viable solution to the domain wall problem.
• The proposed mechanism has significant cosmological implications for axion models.
• Complementary axion detection strategies, including those that preserve and violate axion quality, are strongly supported by these findings.