Three Loop Analysis of the Critical $O(N)$ Models in $6-\epsilon$ Dimensions

TL;DR

This paper performs a three-loop analysis of the $O(N)$ models in $6- ext{epsilon}$ dimensions, calculating beta functions, operator dimensions, and critical values of $N$, revealing new fixed points and connections to non-unitary models.

Contribution

It provides the first three-loop beta functions for the $O(N)$ models in $6- ext{epsilon}$ dimensions and explores their fixed points and operator dimensions, including the $N=1$ case with $Z_2$ symmetry.

Findings

Significant reduction in critical $N$ as dimension decreases to 5.

Existence of an IR stable fixed point at imaginary couplings for $N=1$.

Connection to non-unitary minimal models in two dimensions.

Abstract

We continue the study, initiated in arXiv:1404.1094, of the $O(N)$ symmetric theory of $N+1$ massless scalar fields in $6-\epsilon$ dimensions. This theory has cubic interaction terms $\frac{1}{2}g_1 \sigma (\phi^i)^2 + \frac{1}{6}g_2 \sigma^3$. We calculate the 3-loop beta functions for the two couplings and use them to determine certain operator scaling dimensions at the IR stable fixed point up to order $\epsilon^3$. We also use the beta functions to determine the corrections to the critical value of $N$ below which there is no fixed point at real couplings. The result suggests a very significant reduction in the critical value as the dimension is decreased to $5$. We also study the theory with $N=1$, which has a $Z_2$ symmetry under $\phi\rightarrow -\phi$. We show that it possesses an IR stable fixed point at imaginary couplings which can be reached by flow from a nearby fixed point describing a pair of $N=0$ theories. We calculate certain operator scaling dimensions at the IR fixed point of the $N=1$ theory and suggest that, upon continuation to two dimensions, it describes a non-unitary conformal minimal model.