Gentle introduction to rigorous Renormalization Group: a worked fermionic example

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 $\epsilon$ 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 ${\varphi }^4$ 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 $\epsilon$, a somewhat surprising fact relying on the fermionic nature of the problem.

55 pages + appendices, many figures; v2: refs added; v3: refs added, intro greatly expanded, App. J.1 added, version accepted by JHEP; v4: minor misprints corrected

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• Critical Field Theories with OSp$(1|2M)$ Symmetry
• Finite-size versus finite-temperature effects in the critical long-range $O(N)$ model
• Long-range fermions and critical dualities
• A Lorentzian renormalisation group equation for gauge theories
• Almost optimal upper bound for the ground state energy of a dilute Fermi gas via cluster expansion
• Pressure of a dilute spin-polarized Fermi gas: Lower bound
• Percolation transition for random forests in $d\ge 3$
• Constructing a weakly-interacting fixed point of the Fermionic Polchinski equation
• The scaling limit of boundary spin correlations in non-integrable Ising models