Axion Dark Matter: What is it and Why Now?

TL;DR

This paper reviews the axion as a promising dark matter candidate, discussing its theoretical origins, production in the early universe, and the experimental efforts to detect it in the near future.

Contribution

It provides a comprehensive overview of the axion's theoretical background, its role as dark matter, and recent experimental prospects for detection.

Findings

Astrophysical constraints narrow the axion mass and interaction range.

The axion's properties are linked to its potential detectability in upcoming experiments.

Historical and theoretical context supports ongoing experimental searches.

Abstract

The axion has emerged in recent years as a leading particle candidate to provide the mysterious dark matter in the cosmos, as we review here for a general scientific audience. We describe first the historical roots of the axion in the Standard Model of particle physics and the problem of charge-parity invariance of the strong nuclear force. We then discuss how the axion emerges as a dark matter candidate, and how it is produced in the early Universe. The symmetry properties of the axion dictate the form of its interactions with ordinary matter. Astrophysical considerations restrict the particle mass and interaction strengths to a limited range, which facilitates the planning of experiments to detect the axion. A companion review discusses the exciting prospect that the axion could {indeed} be detected in the near term in the laboratory.