Holography for QCD(Adj) and QCD(Adj)+F

TL;DR

This paper explores the holographic description of confinement and chiral symmetry breaking in SU(Nc) gauge theories with adjoint fermions, proposing models that suggest possible scale separation between these phenomena and encouraging lattice investigations.

Contribution

It introduces a holographic framework for analyzing IR dynamics of adjoint QCD-like theories, including the effects of adding fundamental fermions to separate confinement and chiral symmetry breaking scales.

Findings

Holographic models indicate potential scale separation between confinement and chiral symmetry breaking.

Adding fundamental fermions can significantly enlarge the scale gap, up to an order of magnitude.

Results challenge lattice studies to verify the existence or absence of such scale gaps.

Abstract

We discuss confinement and chiral symmetry breaking in SU(Nc) gauge theories with fermions in the adjoint representation. There has been considerable work on studying these theories compactified on a small circle (with compactification scale 1/L large relative to the strong coupling scale of the theory). The weakly coupled IR theory of photons exhibits confinement through a density of magnetically charged instanton configurations. As the compactification scale 1/L approaches the strong coupling scale, the IR theory becomes strongly coupled. In this regime we propose a holographic description of the IR degrees of freedom. The instanton condensation scale can be associated with a scale at which the Brietenlohner-Freedman (BF) bound is violated in the model and the glueball spectrum computed. We can also introduce the adjoint fermions which holographically display a BF bound violation associated to their running anomalous dimension. Very naively extending the perturbative results to the non-perturbative regime suggests that chiral symmetry breaking might occur ahead of confinement, but equally they may be joined phenomena. If the two phenomena are separate, then it would be useful to be able to enlarge the gap in scales. We propose adding fermions in the fundamental representation as well, which in a holographic model (that favours this separation) can greatly enlarge the gap to an order of magnitude. These results challenge the lattice community to seek such scale gaps (or their absence) to further understand the confining and chiral symmetry breaking dynamics.