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 ma 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.
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