$\theta$ dependence of 4D $SU(N)$ gauge theories in the large-$N$ limit

TL;DR

This paper investigates how the topological properties of 4D $SU(N)$ gauge theories and 2D $CP^{N-1}$ models depend on the parameter $ heta$ in the large-$N$ limit, using numerical simulations and analytic calculations.

Contribution

It provides numerical evidence for the large-$N$ scaling of topological coefficients in 4D $SU(N)$ gauge theories and analytically confirms similar behavior in 2D $CP^{N-1}$ models.

Findings

The coefficient $b_2$ scales as $1/N^2$ with $ar{b}_2=-0.23(3)$.

Large-$N$ scaling applies to both 4D $SU(N)$ gauge theories and 2D $CP^{N-1}$ models.

Numerical simulations support the predicted large-$N$ behavior of topological coefficients.

Abstract

We study the large-$N$ scaling behavior of the $\theta$ dependence of the ground-state energy density $E(\theta)$ of four-dimensional (4D) $SU(N)$ gauge theories and two-dimensional (2D) $CP^{N-1}$ models, where $\theta$ is the parameter associated with the Lagrangian topological term. We consider its $\theta$ expansion around $\theta=0$, $E(\theta)-E(0) = {1\over 2}\chi \,\theta^2 ( 1 + b_2 \theta^2 + b_4\theta^4 +\cdots)$ where $\chi$ is the topological susceptibility and $b_{2n}$ are dimensionless coefficients. We focus on the first few coefficients $b_{2n}$, which parametrize the deviation from a simple Gaussian distribution of the topological charge at $\theta=0$. We present a numerical analysis of Monte Carlo simulations of 4D $SU(N)$ lattice gauge theories for $N=3,\,4,\,6$ in the presence of an imaginary $\theta$ term. The results provide a robust evidence of the large-$N$ behavior predicted by standard large-$N$ scaling arguments, i.e. $b_{2n}= O(N^{-2n})$. In particular, we obtain $b_2=\bar{b}_2/N^2 + O(1/N^4)$ with $\bar{b}_2=-0.23(3)$. We also show that the large-$N$ scaling scenario applies to 2D $CP^{N-1}$ models as well, by an analytic computation of the leading large-$N$ dependence.