The Strong CP Problem and Axions

TL;DR

This paper reviews the strong CP problem in QCD, discusses the Peccei-Quinn solution involving axions, and explores the properties, dynamics, and astrophysical constraints of invisible axion models.

Contribution

It provides a detailed analysis of axion properties and their role in solving the strong CP problem, emphasizing invisible axion models and their astrophysical implications.

Findings

Invisible axions are very light, weakly coupled, and long-lived.

Astrophysical and cosmological bounds constrain axion properties.

The Peccei-Quinn mechanism offers a viable solution to the strong CP problem.

Abstract

I describe how the QCD vacuum structure, necessary to resolve the $U(1)_A$ problem, predicts the presence of a P, T and CP violating term proportional to the vacuum angle $\bar{\theta}$. To agree with experimental bounds, however, this parameter must be very small $(\bar{\theta} \leq 10^{-9}$). After briefly discussing some possible other solutions to this, so-called, strong CP problem, I concentrate on the chiral solution proposed by Peccei and Quinn which has associated with it a light pseudoscalar particle, the axion. I discuss in detail the properties and dynamics of axions, focusing particularly on invisible axion models where axions are very light, very weakly coupled and very long-lived. Astrophysical and cosmological bounds on invisible axions are also briefly touched upon.