Hamiltonian Analysis of Gauged $CP^1$ Model, the Hopf term, and fractional spin

TL;DR

This paper performs a detailed Hamiltonian analysis of the gauged $CP^1$ model with a Hopf term, revealing its gauge structure, fractional spin properties, and reductions to simpler models with abelian charges.

Contribution

It provides the first detailed Hamiltonian analysis of the gauged $CP^1$ model with a Hopf term, clarifying its gauge invariance and fractional spin dependence.

Findings

The model has only $SU(2)$ gauge invariance, not $SU(2) imes U(1)$.

The Hopf term induces fractional spin depending on soliton number and nonabelian charge.

Reduced model shows fractional spin depends solely on abelian charge.

Abstract

Recently it was shown by Cho and Kimm that the gauged $CP^1$ model, obtained by gauging the global $SU(2)$ group and adding a corresponding Chern-Simons term, has got its own soliton. These solitons are somewhat distinct from those of pure $CP^1$ model as they cannot always be characterised by $\pi_2(CP^1)=Z$. In this paper, we first carry out a detailed Hamiltonian analysis of this gauged $CP^1$ model. This reveals that the model has only $SU(2)$ as the gauge invariance, rather than $SU(2) \times U(1)$. The $U(1)$ gauge invariance of the original (ungauged) $CP^1$ model is actually contained in the $SU(2)$ group itself. Then we couple the Hopf term associated to these solitons and again carry out its Hamiltonian analysis. The symplectic structures, along with the structures of the constraints of these two models (with or without Hopf term) are found to be essentially the same. The model with a Hopf term is shown to have fractional spin which, when computed in the radiation gauge, is found to depend not only on the soliton number $N$, but also on the nonabelian charge. We then carry out a reduced (partially) phase space analysis in a different physical sector of the model where the degrees of freedom associated with the $CP^1$ fields are transformed away. The model now reduces to a $U(1)$ gauge theory with two Chern-Simons gauge fields getting mass-like terms and one remaining massless. In this case the fractional spin is computed in terms of the dynamical degrees of freedom and shown to depend purely on the charge of the surviving abelian symmetry. Although this reduced model is shown to have its own solitonic configuration, it turns out to be trivial.