Resonant excitation of the axion field during the QCD phase transition

TL;DR

This paper demonstrates that primordial plasma fluctuations during the QCD phase transition can resonantly excite axions, significantly affecting their momentum distribution and abundance, especially for axion masses above a few microelectronvolts.

Contribution

It introduces a mechanism for resonant excitation of axions during the QCD transition, deriving the resulting momentum distribution and quantifying the fraction of excited cold axions.

Findings

Resonant excitation pumps axions from low to higher momentum modes.

The excited fraction of axions can be of order one for masses above a few μeV.

The effect occurs regardless of the timing of inflation relative to the Peccei-Quinn transition.

Abstract

We find that the adiabatic fluctuations produced in the primordial plasma by cosmological inflation resonantly excite the axion field during the QCD phase transition by pumping axions from low momentum modes to modes with momentum up to of order $\sqrt{3} m$ where $m$ is the axion mass. We derive the momentum distribution of the excited axions. The fraction of cold axions that get excited is of order one if the axion mass is larger than a few $\mu$eV. The effect occurs whether inflation happens before or after the Peccei-Quinn phase transition.