QCD axion dark matter from long-lived domain walls during matter domination

TL;DR

This paper explores how long-lived domain walls during a matter-dominated epoch can decay into axions, potentially explaining dark matter without overproduction issues, thus expanding viable parameter space.

Contribution

It demonstrates that matter domination at MeV temperatures allows for successful axion dark matter production from decaying domain walls with less tuning.

Findings

Viable parameter space for axion dark matter is expanded during matter domination.

Domain walls decay during matter domination, reducing axion overproduction.

Explicit symmetry breaking phase tuning constraints are relaxed.

Abstract

The domain wall problem of the Peccei-Quinn mechanism can be solved if the Peccei-Quinn symmetry is explicitly broken by a small amount. Domain walls decay into axions, which may account for dark matter of the universe. This scheme is however strongly constrained by overproduction of axions unless the phase of the explicit breaking term is tuned. We investigate the case where the universe is matter-dominated around the temperature of the MeV scale and domain walls decay during this matter dominated epoch. We show how the viable parameter space is expanded.