Alternative dark matter candidates: Axions

TL;DR

This paper reviews axions as a well-motivated dark matter candidate, discussing their production, properties, and current experimental search efforts, emphasizing their role as cold, collisionless particles fitting large scale structure.

Contribution

It provides a comprehensive overview of axion dark matter predictions and summarizes the latest experimental efforts to detect axions.

Findings

Axions with decay constants >10^9 GeV are viable cold dark matter candidates.

Axions behave as collisionless, stable particles at cosmological scales.

Current experiments are actively searching for axion dark matter signals.

Abstract

The axion is arguably one of the best motivated candidates for dark matter. For a decay constant greater than about 10^9 GeV, axions are dominantly produced non-thermally in the early universe and hence are "cold", their velocity dispersion being small enough to fit to large scale structure. Moreover, such a large decay constant ensures the stability at cosmological time scales and its behaviour as a collisionless fluid at cosmological length scales. Here, we review the state of the art of axion dark matter predictions and of experimental efforts to search for axion dark matter in laboratory experiments.