Towards a Post-Inflationary Composite Axion Model

TL;DR

This paper introduces two novel composite axion models with effective domain wall number one, addressing post-inflationary cosmological issues like relic dilution and axion dark matter production.

Contribution

It proposes two specific composite axion models with N_DW=1 and analyzes their cosmological viability and relic dilution mechanisms.

Findings

Models successfully achieve N_DW=1, avoiding domain wall problems.

Axion strings can re-enter the horizon before or after QCD transition.

Axion dark matter can be produced from string-wall decay.

Abstract

Composite axions offer a scenario where the axion emerges as a pion-like state, avoiding fine-tuning of elementary scalars and ameliorating the axion quality problem. Despite these advantages, their post-inflationary cosmology remains largely unexplored, with challenges including the domain wall problem and the presence of exotic relics. We propose two composite axion models with an effective domain wall number $N_\text{DW} = 1$ and study the dilution of relics via a short period of inflation. One model is based on an $SU(5)$ chiral gauge theory, while the other employs a ``gauged'' $U(1)$ Peccei-Quinn symmetry in vector-like $SU(N)$ gauge theories. We identify the viable parameter space in which axion strings re-enter the horizon before or even after the QCD transition and axion dark matter is dominantly produced from the decay of the string-wall network.