Breakdown of chiral perturbation theory for the axion hot dark matter bound

TL;DR

The paper demonstrates that the standard axion hot dark matter bound is unreliable due to the breakdown of chiral perturbation theory in the relevant regime, and proposes lattice QCD as a more robust approach.

Contribution

It critically assesses the validity of the hot dark matter bound on axion mass and introduces lattice QCD as a promising method for accurate determination.

Findings

Chiral perturbation theory breaks down in the relevant regime for axion HDM bounds.

Next-to-leading order calculations reveal limitations of previous bounds.

Lattice QCD is proposed for reliable axion mass constraints.

Abstract

We show that the commonly adopted hot dark matter (HDM) bound on the axion mass $m_a \lesssim$ 1 eV is not reliable, since it is obtained by extrapolating the chiral expansion in a region where the effective field theory breaks down. This is explicitly shown via the calculation of the axion-pion thermalization rate at the next-to-leading order in chiral perturbation theory. We finally advocate a strategy for a sound extraction of the axion HDM bound via lattice QCD techniques.