Barrier penetration in 1+1-dimensional O(n) sigma models

TL;DR

This paper investigates barrier penetration and tunneling phenomena in 1+1-dimensional O(n) sigma models, revealing new insights into their phase transitions, soliton configurations, and the mass gap at different theta angles.

Contribution

It introduces a novel analysis of the configuration space, corrects previous misconceptions about potential energy and distance, and computes tunneling amplitudes affecting phase transitions.

Findings

Potential energy barriers exist with soliton configurations on them.

Tunneling drives the phase transition at a critical coupling in the O(2) model.

Tunneling paths with half-integer topological charge relate to the massless phase at θ=π.

Abstract

The O(n) nonlinear sigma model in 1+1 dimensions is examined as quantum mechanics on an infinite-dimensional configuration space. Two metrics are defined in this space. One of these metrics is the same as Feynman's distance, but we show his conclusions concerning potential energy versus distance from the classical vacuum are incorrect. The potential-energy functional is found to have barriers; the configurations on these barriers are solitons of an associated sigma model with an external source. The tunneling amplitude is computed for the O(2) model and soliton condensation is shown to drive the phase transition at a critical coupling. We find the tunneling paths in the configuration space of the O(3) model and argue that these are responsible for the mass gap at $\theta=0$. These tunneling paths have half-integer topological charge, supporting the conjecture due to Affleck and Haldane that there is a massless phase at $\theta=\pi$.