Schwinger Dyson Summation of Perturbation Expansions

TL;DR

This paper introduces a hierarchical approximation system for summing perturbation expansions in quantum field theory and condensed matter physics, approaching the full Schwinger Dyson hierarchy as the limit.

Contribution

It presents a novel hierarchical framework of nonlinear equations for correlation functions that approximates the Schwinger Dyson hierarchy, with convergence arguments for finite-dimensional integrals.

Findings

Hierarchy converges to exact solutions as K increases

No finite order captures effects from non-leading saddle points

Potential applications to critical exponents and phase structures

Abstract

We introduce a hierarchical system of approximations for summing both conventional perturbation theory and large N vector expansions of models in quantum field theory and condensed matter physics. Each stage of the hierarchy consists of a closed set of nonlinear equations for one particle irreducible correlation functions with no more than K points and captures the perturbation expansion of each of those functions up to some finite order. As K goes to infinity the full Schwinger Dyson hierarchy is approached. We present an argument that for ordinary finite dimensional integrals, the procedure converges to the exact answer in this limit, despite the fact that no finite order of the hierarchy sees effects from non-leading saddle points of the integral. Some potential applications to the calculation of critical exponents, to low dimensional condensed matter systems, and to the phase structure of the homogeneous electron gas are outlined, but no detailed calculations are presented.