Improved hot dark matter bound on the QCD axion

TL;DR

This paper refines the cosmological constraints on the QCD axion mass by solving detailed Boltzmann equations and using experimental data, setting a new upper limit of 0.24 eV, and highlights the need for advanced calculations for future improvements.

Contribution

It introduces a momentum-dependent Boltzmann equation approach and uses pion scattering data to tighten the axion mass bound beyond previous estimates.

Findings

New upper bound on axion mass: 0.24 eV

Improved modeling of axion-pion interactions

Identifies need for non-perturbative QCD calculations

Abstract

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 $m_a\leq 0.24~\text{eV}$. To further improve the bound and exploit the reach of upcoming cosmological surveys, reliable non-perturbative calculations above the QCD crossover are needed.