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

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.

Comments: 7 pages, 4 figures. Title changed. To appear in Physical Review Letters