RG flows in 2d QCD

TL;DR

This paper analyzes the infrared behavior of 2d QCD, identifying the effective theories, operator mappings, and the effects of deformations, including evidence for dynamical symmetry breaking and fermion condensates.

Contribution

It explicitly constructs the RG flow map in 2d QCD, classifies IR operators, and explores nonperturbative phenomena like fermion condensates and symmetry breaking.

Findings

IR theories are either CFTs or TQFTs depending on the spectrum

Operators creating massless excitations remain in IR with large anomalous dimensions

Evidence for nonperturbative fermion condensates causing gauge symmetry breaking

Abstract

2d QCD, Yang-Mills theory with gauge group G and massless quarks in representations (R_\ell, R_r) of G, flows in the infrared to a CFT or a TQFT depending on whether spectrum is gapless or gapped. We identify the infrared effective theory and construct the explicit RG flow map between the QCD operators in the UV and the IR, in particular identifying which operators create massive excitations and decouple in the IR, and those that create massless excitations or vacua and remain as nontrivial IR operators. We determine the exact scaling dimensions of the QCD operators that remain in the IR, which generically acquire large anomalous dimensions. We also study QCD deformed by various operators in the ultraviolet (such as mass terms or four-fermi interactions), and determine the theory that emerges at low energies. We conjecture and provide some evidence for QCD deformed by various operators developing a nonperturbative fermion condensate that dynamically breaks the gauge symmetry G, thus explicitly realizing an old idea known as "tumbling" in an exactly soluble setting.