$Z_2$ Massive Axions, Domain Walls and Inflation

TL;DR

This paper investigates a $Z_2$-broken $U(1)$ axion model, analyzing domain wall properties and inflationary scenarios, providing analytical solutions and insights into parameter effects relevant for cosmology.

Contribution

It introduces a $Z_2$-broken $U(1)$ axion model with two stable domain walls and offers analytical solutions for domain wall properties and inflationary dynamics.

Findings

Two stable domain walls in the model.

Analytical approximation for domain wall surface energy density.

Successful inflationary scenarios require large field values.

Abstract

We have analyzed a $U(1)$ model which is broken explicitly to a $Z_2$ model. The model results in generating two stable domain walls, in contrast with the more common $N_{DW}=1$ version which is prevalently used to explain axion invisibility for $U_{PQ}(1)$ model. We have been meticulous to take into account any possible relation with previous studies, even if they apparently belong to different lines of research. Then we have scrutinized the domain wall properties of the model,proposing a rigorous approximate solution which fully satisfies boundary conditions and the static virial theorem simultaneously. Invoking the mentioned approximation, we have been able to obtain an analytical insight about the effect of parameters on the domain wall features, particularly on their surface energy density which is of great importance in cosmological studies when one tries to avoid domain wall energy domination problem. Next, we have mainly focused on the likely inflationary scenarios of the model, including saddle point inflation, again insisting on analytical discussions to be able to follow the role of parameters. We have tried to categorize any inflationary scenario into the known categories to take advantage of the previous detailed studies under the inflationary topic over the decades. We have concluded that any successful inflationary scenario requires large fields definition for the model. Calculations are mainly done analytically and numerical results are used as supportive material.