Axions at the meV Crossroads: Theory, Cosmology, Astrophysics, and Experiments

TL;DR

This comprehensive review explores the theoretical motivations, cosmological and astrophysical signatures, and experimental strategies for detecting meV mass axions, emphasizing their significance in testing beyond-Standard-Model physics and dark matter models.

Contribution

It provides an integrated overview of meV axion research, highlighting recent advances, open questions, and the convergence of theoretical, cosmological, astrophysical, and experimental efforts.

Findings

MeV axions are well-motivated theoretically and have significant cosmological impacts.

Astrophysical environments like stars and supernovae are effective production sites for axions.

A diverse array of experimental approaches is being developed to detect meV axions.

Abstract

The meV mass range has emerged as a focal point in axion physics, where advances in theory, cosmology, astrophysics, and experimental techniques converge. Axions in this mass range are theoretically well motivated, can arise in ultraviolet-complete models, and can have significant cosmological impacts as dark matter or dark radiation. In parallel, their efficient production in stellar and supernova environments provides powerful astrophysical probes. Here, we provide a comprehensive overview of meV axions across these domains, highlighting both established results and open questions. We discuss the theoretical underpinnings of meV axions, their cosmological and astrophysical signatures, and the diverse experimental strategies -- ranging from helioscopes and haloscopes to quasiparticle systems and large-volume Cherenkov detectors -- that aim to explore this regime. The convergence of these approaches emphasizes the pivotal role of the meV mass range for axion discovery in the coming years, identifying meV axions as a key probe for testing beyond-Standard-Model physics. This review document is the direct outcome of the discussions at the dedicated workshop "The meV Mass Axion Frontier: Challenges and Opportunities", held at Laboratori Nazionali di Frascati (IT) on 27--28 October 2025, and organized by the EU funded COST Action "Cosmic WISPers in the Dark Universe: Theory, astrophysics, and experiments" (CA21106, https://www.cost.eu/actions/CA21106). Its aim is to provide an overview of current efforts in meV axion research, their motivations, and the research goals that animate the community involved in this search.