Zonotopes and four-dimensional superconformal field theories

TL;DR

This paper proves the existence and uniqueness of the a-maximization solution for a class of superconformal theories using zonotope volumes, establishing bounds and monotonicity properties related to toric diagrams.

Contribution

It introduces a geometric interpretation of a-maximization via zonotope volumes and proves the uniqueness of the critical point for a broad class of theories.

Findings

The a-function has a unique global maximum for these theories.

A universal upper bound for R-charges is established.

The a-function decreases as the toric diagram shrinks.

Abstract

The a-maximization technique proposed by Intriligator and Wecht allows us to determine the exact R-charges and scaling dimensions of the chiral operators of four-dimensional superconformal field theories. The problem of existence and uniqueness of the solution, however, has not been addressed in general setting. In this paper, it is shown that the a-function has always a unique critical point which is also a global maximum for a large class of quiver gauge theories specified by toric diagrams. Our proof is based on the observation that the a-function is given by the volume of a three dimensional polytope called "zonotope", and the uniqueness essentially follows from Brunn-Minkowski inequality for the volume of convex bodies. We also show a universal upper bound for the exact R-charges, and the monotonicity of a-function in the sense that a-function decreases whenever the toric diagram shrinks. The relationship between a-maximization and volume-minimization is also discussed.