Gentle introduction to rigorous Renormalization Group: a worked fermionic example

TL;DR

This paper provides a rigorous, non-perturbative analysis of the Renormalization Group fixed point in a fermionic model with long-range interactions, serving as a benchmark for approximation methods in critical phenomena.

Contribution

It introduces a fully rigorous and constructive approach to identifying RG fixed points in a fermionic model, including a detailed Banach space framework and an analytic dependence on parameters.

Findings

Identified the Banach space of interactions containing relevant and irrelevant terms.

Constructed a convergent approximation scheme for the fixed point.

Proved the fixed point's analyticity in the parameter ε.

Abstract

Much of our understanding of critical phenomena is based on the notion of Renormalization Group (RG), but the actual determination of its fixed points is usually based on approximations and truncations, and predictions of physical quantities are often of limited accuracy. The RG fixed points can be however given a fully rigorous and non-perturbative characterization, and this is what is presented here in a model of symplectic fermions with a nonlocal ("long-range") kinetic term depending on a parameter $\varepsilon$ and a quartic interaction. We identify the Banach space of interactions, which the fixed point belongs to, and we determine it via a convergent approximation scheme. The Banach space is not limited to relevant interactions, but it contains all possible irrelevant terms with short-ranged kernels, decaying like a stretched exponential at large distances. As the model shares a number of features in common with $\phi^4$ or Ising models, the result can be used as a benchmark to test the validity of truncations and approximations in RG studies. The analysis is based on results coming from Constructive RG to which we provide a tutorial and self-contained introduction. In addition, we prove that the fixed point is analytic in $\varepsilon$, a somewhat surprising fact relying on the fermionic nature of the problem.