Revisiting the Minimal Nelson-Barr Model

TL;DR

This paper improves the minimal Nelson-Barr model for solving the strong CP problem by introducing an approximate global symmetry, addressing the quality and domain wall issues, and demonstrating the viability of thermal leptogenesis.

Contribution

It proposes a simple solution to the quality and domain wall problems in the minimal Nelson-Barr model and confirms the feasibility of thermal leptogenesis.

Findings

Imposing an approximate global symmetry solves the quality problem.

A simple solution to the cosmological domain wall problem is proposed.

Thermal leptogenesis remains viable in the revised model.

Abstract

We revisit the minimal Nelson-Barr model for solving the strong CP problem through the idea of spontaneous CP breaking. The minimal model suffers from the quality problem, which means that the strong CP angle is generated by higher-dimensional operators and one-loop effects. Consequently, it has been considered that there is a cosmological domain wall problem and that leptogenesis does not work. We point out that just imposing an additional approximate global symmetry solves the quality problem. We also propose a simple solution to the domain wall problem and show that the thermal leptogenesis scenario works.