Baryons as relativistic three-quark bound states

TL;DR

This paper reviews the theoretical understanding of baryons as relativistic three-quark bound states within QCD, highlighting recent progress, challenges, and the connection to experimental data and lattice QCD.

Contribution

It provides a systematic overview of nonperturbative QCD approaches, especially Dyson-Schwinger and Bethe-Salpeter equations, and compares results with lattice QCD and quark models.

Findings

Confronts Dyson-Schwinger results with lattice QCD data.

Analyzes baryon spectrum and form factors.

Discusses two-photon processes and Compton scattering.

Abstract

We review the spectrum and electromagnetic properties of baryons described as relativistic three-quark bound states within QCD. The composite nature of baryons results in a rich excitation spectrum, whilst leading to highly non-trivial structural properties explored by the coupling to external (electromagnetic and other) currents. Both present many unsolved problems despite decades of experimental and theoretical research. We discuss the progress in these fields from a theoretical perspective, focusing on nonperturbative QCD as encoded in the functional approach via Dyson-Schwinger and Bethe-Salpeter equations. We give a systematic overview as to how results are obtained in this framework and explain technical connections to lattice QCD. We also discuss the mutual relations to the quark model, which still serves as a reference to distinguish 'expected' from 'unexpected' physics. We confront recent results on the spectrum of non-strange and strange baryons, their form factors and the issues of two-photon processes and Compton scattering determined in the Dyson-Schwinger framework with those of lattice QCD and the available experimental data. The general aim is to identify the underlying physical mechanisms behind the plethora of observable phenomena in terms of the underlying quark and gluon degrees of freedom.