Stable $Z$-strings with topological polarization in two Higgs doublet model

TL;DR

This paper demonstrates the stability of a novel vortex molecule configuration of Z-strings with topological polarization in the two Higgs doublet model, extending the understanding of topological defects beyond the Standard Model.

Contribution

It introduces a new stable vortex molecule configuration of Z-strings with fractional fluxes and topological polarization in the two Higgs doublet model, expanding the known stable solutions.

Findings

Vortex molecule is stable over a wide parameter space.

The configuration involves topological polarization with fractional fluxes.

Stability persists beyond the Standard Model parameter range.

Abstract

We find that a $Z$-string is stable in a wide range of parameter space of the two Higgs doublet model due to a split into a pair of two topological $Z$-strings with fractional $Z$ fluxes. This configuration, a bound state of the two strings connected by a domain wall, is called a vortex molecule. Although the vortex molecule has no net topological charge, the locally induced topological charge density is polarized, namely distributed positively around one constituent string and negatively around the other constituent string, leading to the stability of the molecule. We numerically show that the vortex molecule is indeed a stable solution of the equation of motions in a much wider parameter space of the model than the usual axially symmetric $Z$-string in the Standard Model and the two Higgs doublet model, although it is not the case for experimental values of the parameters.