Collapse of Axion Domain Wall Induced by Helical Primordial Magnetic Fields

TL;DR

This paper proposes a novel mechanism where helical primordial magnetic fields induce the decay of axion domain walls, potentially resolving the overclosure problem without breaking PQ symmetry, supported by lattice simulations and analysis of magnetic field effects.

Contribution

It introduces a symmetry-preserving decay mechanism for axion domain walls via coupling to helical magnetic fields, validated through 3D lattice simulations.

Findings

Magnetic field correlation length influences domain wall decay rate.

The mechanism produces axion and gravitational wave radiation.

Simulations confirm successful domain wall decay under this coupling.

Abstract

Stable domain wall (DW) must decay to avoid overclose the Universe. A commonly used solution is to slightly break the PQ symmetry by introducing a bias term in the potential. In this work, we propose an alternative, symmetry-preserving mechanism: coupling the axion field to a helical primordial magnetic field (PMF) via the Chern-Simons term. Using three-dimensional lattice simulations, we evolve the DW network and demonstrate that it can successfully drive DW decay. Our quantitative results further show that the correlation length of the PMF plays a crucial role in determining the decay rate of the DW network and the resulting axion and gravitational wave radiation.