Scale without conformal invariance in membrane theory

TL;DR

This paper investigates the scale and conformal symmetries in membrane theory, showing that the infrared fixed point exhibits only scale invariance without full conformal invariance due to the presence of a virial current.

Contribution

It demonstrates that in a membrane model, the infrared fixed point is only scale-invariant and not conformally invariant, highlighting the role of symmetries and virial currents.

Findings

Infrared fixed point is only scale-invariant, not conformally invariant.

Presence of a non-vanishing virial current with scaling dimension D-1.

Symmetries under translations and rotations protect certain operators from anomalous dimensions.

Abstract

We investigate the relation between dilatation and conformal symmetries in the statistical mechanics of flexible crystalline membranes. We analyze, in particular, a well-known model which describes the fluctuations of a continuum elastic medium embedded in a higher-dimensional space. In this theory, the renormalization group flow connects a non-interacting ultraviolet fixed point, where the theory is controlled by linear elasticity, to an interacting infrared fixed point. By studying the structure of correlation functions and of the energy-momentum tensor, we show that, in the infrared, the theory is only scale-invariant: the dilatation symmetry is not enhanced to full conformal invariance. The model is shown to present a non-vanishing virial current which, despite being non-conserved, maintains a scaling dimension exactly equal to $D - 1$, even in presence of interactions. We attribute the absence of anomalous dimensions to the symmetries of the model under translations and rotations in the embedding space, which are realized as shifts of phonon fields, and which protect the renormalization of several non-invariant operators. We also note that closure of a symmetry algebra with both shift symmetries and conformal invariance would require, in the hypothesis that phonons transform as primary fields, the presence of new shift symmetries which are not expected to hold on physical grounds. We then consider an alternative model, involving only scalar fields, which describes effective phonon-mediated interactions between local Gaussian curvatures. The model is described in the ultraviolet by two copies of the biharmonic theory, which is conformal, but flows in the infrared to a fixed point which we argue to be only dilatation-invariant.