Energy field of critical Ising model and examples of singular fields in QFT

TL;DR

This paper proves the singularity of certain quantum field theory (QFT) fields related to the critical Ising model and hierarchical models, indicating these fields do not exist as regular distributions and highlighting their complex nature.

Contribution

It demonstrates the singularity of three natural fields in QFT, including the energy field of the 2D Ising model and hierarchical sine-Gordon and 4 fields, using a novel approach applicable to various settings.

Findings

The energy field of the 2D Ising model is locally singular.

Hierarchical sine-Gordon field is singular relative to the Gaussian Free Field.

Hierarchical 4_3 field is singular relative to the 3D GFF.

Abstract

The goal of this paper is to prove singularity of three natural fields in QFT with respect to their natural base measure. The fields we consider are the following ones: (1) The near-critical limit of the $2d$ Ising model (in the $\beta$-direction) is locally singular w.r.t the critical scaling limit of $2d$ Ising. (N.B. In the $h$-direction it is not locally singular). (2) The $2d$ Hierarchical Sine-Gordon field is singular w.r.t the $2d$ hierarchical Gaussian Free Field for all $\beta\in[\beta_{L^2}, \beta_{BKT})$. (3) The Hierarchical $\Phi^4_3$ field is singular w.r.t the $3d$ hierarchical GFF. Item (1) gives the first strong indication that the energy field of critical $2d$ Ising model does not exist as a random Schwarz distribution on the plane. Item (2) has been proved to be singular for the non-hierarchical $2d$ Sine-Gordon sufficiently far from the BKT point in [GM24] while item (3) is proved to be singular for the non-hierarchical $3d$ $\Phi^4_3$ field in [BG21, OOT21, HKN24]. We believe our way to detect a singular behaviour at all scales is very much down to earth and may be applicable in all settings where one has a good enough control on the so-called effective potentials.