Infrared properties of two-dimensional $\mathrm{SU}(N)/H$ nonlinear $\sigma$ models at nonzero $\theta$ angles

TL;DR

This paper develops a strategy to analyze the low-energy behavior of two-dimensional nonlinear sigma models with theta terms, applying it to models with various target spaces and revealing their phase structures and RG flows.

Contribution

It introduces a general approach to study these models' low-energy properties and demonstrates how different target spaces lead to distinct RG flows and phase behaviors.

Findings

Flag-manifold models flow to SU(N)$_1$ fixed point

C$P^{N-1}$ and Grassmannian models at $ heta= ext{pi}$ are massive with two-fold degeneracy

Method links symmetry, anomalies, and conformal field theory to nonlinear sigma models

Abstract

A general strategy is proposed to explore the low-energy properties of two-dimensional nonlinear $\sigma$ models with $\theta$ terms. We demonstrate its application to nonlinear $\sigma$ models with the target space $\text{SU($N$)}$/H, which include $\mathbb{C}P^{N-1}$, complex Grassmannian manifolds as well as the flag $\text{SU($N$)}/\text{U(1)}^{N-1}$ and $\text{SU($N$)})/\text{SO($N$)}$ manifolds. By analyzing the symmetry and its anomaly content, we realize these nonlinear $\sigma$ models through perturbations added to the SU(N)$_1$ conformal field theory. For the flag-manifold $\text{SU($N$)}/\text{U(1)}^{N-1}$ and $\text{SU($N$)})/\text{SO($N$)}$ models, those perturbations are shown to correspond to the marginal current-current operator with the specific sign which leads to a massless renormalization group flow to the SU(N)$_1$ fixed point. In contrast, a massive regime with a two-fold ground-state degeneracy is found for the $\mathbb{C}P^{N-1}$ ($N >2$) and Grassmannian nonlinear $\sigma$ models at $\theta=\pi$.