Topological Charge Asymmetry in a $\mathbb{C}\mathrm{P}^N$ Skyrmion-Fermion Coupled System

TL;DR

This work analyzes a $2+1$ dimensional CP$^N$ Skyrmion coupled to Dirac fermions with a self-consistent backreaction. By enforcing a rotationally symmetric ansatz and solving the coupled bosonic-fermionic equations, the authors reveal a pronounced asymmetry between configurations with topological charges $Q$ and $-Q$ in the strong-coupling regime. The study demonstrates that fermionic backreaction breaks the naive $Q\to -Q$ symmetry of the Lagrangian, producing distinct localization patterns for the fermion density and deformations of the Skyrmion energy density that depend on the sign of $Q$. The findings highlight a physically observable impact of the topological charge sign and lay groundwork for further stability analyses and extensions to general CP$^N$ models.

Abstract

Topology plays a central role in classifying solitonic configurations in field theories, providing robustness and a nonperturbative label, the so-called topological charge $Q$. In soliton-fermion coupled systems, the relation between the topological charge and the number of zero modes is well established through the index theorem. However, the physical consequences of the sign of the topological charge have remained largely unexplored. In this work, we study fermions in $2+1$ dimensions coupled to Skyrmions with target space $\mathbb{C}\mathrm{P}^N$, particularly focusing on the backreactions of the fermions and on the sign of the topological charge. We obtain the solutions in a self-consistent manner, which exhibit an asymmetry with respect to the topological charge $\pm Q$ especially in the strong coupling regimes. This asymmetry is caused from the fermionic eigenvalue problem inherent in the self-consistent formulation. Although the Lagrangian is symmetric under $Q\to-Q$, the coupled equations for the Skyrmions and anti-Skyrmions become inequivalent once fermionic backreaction is taken into account. We demonstrate the mechanism in $\mathbb{C}\mathrm{P}^1$ and $\mathbb{C}\mathrm{P}^2$ Skyrmions, but the analysis is directly extendable for the general $\mathbb{C}\mathrm{P}^N$.

Figures (21)

• Figure 1: Schematic illustration of a spectral flow with solitons $(Q>0)$ and anti-solitons $(Q<0)$.
• Figure 2: Schematic illustration of the self-consistent analysis
• Figure 3: An example of the spectral flow on a background Skyrmion with $\qty(n_1,n_2)=\qty(-1,-3)$.
• Figure 4: $\mathbb{C}\mathrm{P}^1$ case with $Q=1\ \qty(n_1=0,n_2=1)$
• Figure 5: $\mathbb{C}\mathrm{P}^1$ case with $Q=-1\ \qty(n_1=0,n_2=-1)$