Axions and ALPs: a very short introduction

TL;DR

This paper provides a concise overview of axions and ALPs, discussing their theoretical origins, diverse properties, cosmological roles, and experimental signatures, emphasizing their significance as dark matter candidates.

Contribution

It offers a brief, comprehensive review of axion models, their couplings, cosmological implications, and phenomenology, highlighting recent theoretical and observational insights.

Findings

Axions can have a wide range of masses and couplings.

Light axions are viable dark matter candidates.

Distinct cosmological populations of axions exist, influencing structure formation.

Abstract

The QCD axion was originally predicted as a dynamical solution to the strong CP problem. Axion like particles (ALPs) are also a generic prediction of many high energy physics models including string theory. Theoretical models for axions are reviewed, giving a generic multi-axion action with couplings to the standard model. The couplings and masses of these axions can span many orders of magnitude, and cosmology leads us to consider several distinct populations of axions behaving as coherent condensates, or relativistic particles. Light, stable axions are a mainstay dark matter candidate. Axion cosmology and calculation of the relic density are reviewed. A very brief survey is given of the phenomenology of axions arising from their direct couplings to the standard model, and their distinctive gravitational interactions.