Parity solves the Strong CP problem

TL;DR

This paper defends the traditional solutions to the Strong CP problem, emphasizing the stability of P and CP symmetry-based approaches despite recent counterarguments involving initial theta vacuum states.

Contribution

It clarifies the nature of the theta vacuum and reaffirms the validity of P and CP symmetry solutions to the Strong CP problem.

Findings

Reaffirmation of P and CP symmetry solutions

Analysis of the theta vacuum's role in the Strong CP problem

Counterarguments against axionless solutions are addressed

Abstract

A recent paper "What can solve the strong CP problem?" goes counter to conventional wisdom by arguing that the universe was in an initial state that combines different eigenstates of $\theta$ (of the theta vacuum of QCD), and asserts that for such an initial state, axionless solutions to the Strong CP Problem based on P and CP symmetries will not work. We discuss the nature of the theta vacuum in light of this work, and find that the conventional framing and solutions based on P and CP symmetries are on a firm footing.