Derivative Expansions of the Exact Renormalisation Group and SU(N|N) Gauge Theory

TL;DR

This paper analyzes the convergence properties of derivative expansions in the exact renormalisation group, demonstrating convergence at one and two loops with smooth cutoffs, and introduces a finite regularisation scheme for SU(N|N) gauge theories.

Contribution

It provides a detailed study of derivative expansion convergence in the ERG and introduces a novel regularisation scheme for gauge theories using SU(N|N).

Findings

Derivative expansion converges at one loop with certain smooth cutoffs.

Two-loop derivative expansion converges rapidly with exponential cutoff.

A finite regularisation scheme for SU(N|N) gauge theories is proposed.

Abstract

We investigate the convergence of the derivative expansion of the exact renormalisation group, by using it to compute the beta function of scalar theory. We demonstrate that the derivative expansion of the Polchinski flow equation converges at one loop for certain fast falling smooth cutoffs. The derivative expansion of the Legendre flow equation trivially converges at one loop, but also at two loops: slowly with sharp cutoff (as a momentum-scale expansion), and rapidly in the case of a smooth exponential cutoff. We also show that the two loop contributions to certain higher derivative operators (not involved in beta) have divergent momentum-scale expansions for sharp cutoff, but the smooth exponential cutoff gives convergent derivative expansions for all such operators with any number of derivatives. In the latter part of the thesis, we address the problems of applying the exact renormalisation group to gauge theories. A regularisation scheme utilising higher covariant derivatives and the spontaneous symmetry breaking of the gauge supergroup SU(N|N) is introduced and it is demonstrated to be finite to all orders of perturbation theory.