Axion-pion thermalization rate in unitarized NLO chiral perturbation theory

TL;DR

This paper calculates the axion-pion scattering rate using unitarized NLO chiral perturbation theory to improve the accuracy of axion relic density predictions in the early universe.

Contribution

It introduces a unitarized NLO chiral perturbation theory approach to extend the validity of axion-pion scattering calculations for cosmological applications.

Findings

Extended the validity range of axion-pion scattering calculations.

Provided more reliable estimates for axion relic density.

Discussed implications for cosmological observables.

Abstract

We compute the axion-pion scattering $a \pi \to \pi \pi$, relevant for the axion thermalization rate in the early universe, within unitarized NLO chiral perturbation theory. The latter extends the range of validity of the chiral expansion of axion-pion scattering and thus provides a crucial ingredient for the reliable determination of the relic density of thermal axions, whenever the axion decoupling temperature is below that of the QCD phase transition. Implications for cosmological observables are briefly discussed.