Note on a solution to domain wall problem with the Lazarides-Shafi mechanism in axion dark matter models

Abstract

Axion is a promising candidate of dark matter. After the Peccei-Quinn symmetry breaking, axion strings are formed and attached by domain walls when the temperature of the universe becomes comparable to the QCD scale. Such objects can cause cosmological disasters if they are long-lived. As a solution for it, the Lazarides-Shafi mechanism is often discussed through introduction of a new non-Abelian (gauge) symmetry. We study this mechanism in detail and show configuration of strings and walls. Even if Abelian axion strings with a domain wall number greater than one are formed in the early universe, each of them is split into multiple Alice axion strings due to a repulsive force between the Alice strings even without domain wall. When domain walls are formed as the universe cools down, a single Alice string can be attached by a single wall because a vacuum is connected by a non-Abelian rotation without changing energy. Even if an Abelian axion string attached by domain walls are created due to the Kibble Zurek mechanism at the chiral phase transition, such strings are also similarly split into multiple Alice strings attached by walls in the presence of the domain wall tension. Such walls do not form stable networks since they collapse by the tension of the walls, emitting axions.

arXiv comment20 pages, 10 figures

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• String monopoles, string walls, vortex-skyrmions and nexus objects in polar distorted B-phase of ${}^3$He
• Electroweak axion string and superconductivity
• Formation of Chiral Soliton Lattice
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• Aharonov-Bohm defects
• One dimensional nexus objects, network of Kibble-Lazarides-Shafi string walls, and their spin dynamic response in polar distorted B-phase of ${}^3$He
• Resolving the QCD Axion Domain Wall Problem with a Light Axion
• N=2 PNGB Quintessence Dark Energy
• Confinement and moduli locking of Alice strings and monopoles
• Fibration, Nexus and Cosmological Composite Topological Defects in Uniaxially Disordered Superfluid ${}^3$He