Putting the Brakes on Axion Strings: Friction and Its Impact on the QCD Axion Abundance

TL;DR

This paper investigates how friction between the thermal bath and axion strings in DFSZ models affects axion string dynamics, leading to revised predictions for axion masses compatible with dark matter abundance.

Contribution

It introduces the impact of friction on axion string evolution in DFSZ models, showing significant changes in axion abundance predictions for lower decay constants.

Findings

Friction delays axion string scaling onset.

Increased axion energy density due to friction.

Both meV and 0.1 eV axion masses can account for dark matter.

Abstract

A compelling production mechanism for QCD axion dark matter is from the scaling dynamics of early universe axion strings. We show that in DFSZ-like models containing tree-level interactions between fermions and the axion, friction between the thermal bath and the axion string drastically changes the behavior of the axion string network for lower $f_a$ values. Friction delays the onset of scaling and increases the energy density of axions. Once the effects of friction are included, we argue that in addition to the standard value of $m_a \sim$ meV, $m_a \sim 0.1$ eV also reproduces the dark matter energy density.