We strengthen the cosmological bound on the axion mass, by solving the
momentum-dependent Boltzmann equations for axion-pion scatterings and by using
a phenomenological production rate derived from pion-pion scattering data,
overcoming the breakdown of chiral perturbation theory. Using present
cosmological datasets we obtain ${m}_{a}\le 0.24\text{}\text{eV}$ . To further improve
the bound and exploit the reach of upcoming cosmological surveys, reliable
non-perturbative calculations above the QCD crossover are needed.

PREPRINT

# Improved hot dark matter bound on the QCD axion

Alessio Notari, Fabrizio Rompineve, Giovanni Villadoro

Submitted on 7 November 2022

## Abstract

## Preprint

Comment: 6+8 pages, 7 figures

Subjects: High Energy Physics - Phenomenology; Astrophysics - Cosmology and Nongalactic Astrophysics