Domain walls in Nelson-Barr axion model

TL;DR

This paper presents a Nelson-Barr axion model that naturally addresses the strong CP problem, featuring domain wall dynamics and potential bias mechanisms that influence cosmological phenomena like dark matter and gravitational waves.

Contribution

It introduces a novel Nelson-Barr axion model with a built-in domain wall collapse mechanism, connecting strong CP solution with cosmological implications.

Findings

Domain walls can be naturally collapsed by potential bias.

The model predicts gravitational wave signals from domain wall collapse.

Dark matter production is linked to axion dynamics in this framework.

Abstract

We explore a concrete realization of a Nelson-Barr model addressing the strong CP problem with suppressed unfavorable corrections. This model has a scalar field that spontaneously breaks discrete symmetry, and its phase component can naturally be relatively light, which we call the Nelson-Barr axion. It has both a tree-level potential and the QCD instanton-induced potential like the QCD axion, each minimizing at the CP-conserving point. While one potential leads to domain wall formation, the other works as a potential bias. This model provides a natural setup for the collapse of the axion domain walls by a potential bias without spoiling a solution to the strong CP problem. We discuss the cosmological implications of domain wall collapses, including dark matter production and gravitational wave emission.