Baryon Spectroscopy and the Origin of Mass

TL;DR

This paper discusses the challenges in understanding baryon excitation spectra, reviews recent experimental findings, and compares various theoretical models, highlighting the promising agreement with an AdS/QCD derived mass formula.

Contribution

It provides a comprehensive review of baryon spectroscopy, emphasizing the comparison of experimental data with multiple theoretical models, especially the AdS/QCD approach.

Findings

Photoproduction experiments suggest new baryon states.

Best model agreement with AdS/QCD mass formula.

Challenges in interpreting baryon spectra from traditional models.

Abstract

The proton mass arises from spontaneous breaking of chiral symmetry and the formation of constituent quarks. Their dynamics cannot be tested by proton tomography but only by studying excited baryons. However, the number of excited baryons is much smaller than expected within quark models; even worse, the existence of many known states has been challenged in a recent analysis which includes - compared to older analyses - high-precision data from meson factories. Hence $\pi N$ elastic scattering data do not provide a well-founded starting point of any phenomenological analysis of the baryon excitation spectrum. Photoproduction experiments now start to fill in this hole. Often, they confirm the old findings and even suggest a few new states. These results encourage attempts to compare the pattern of observed baryon resonances with predictions from quark models, from models generating baryons dynamically from meson-nucleon scattering amplitudes, from models based on gravitational theories, and with the conjecture that chiral symmetry may be restored at high excitation energies. Best agreement is found with a simple mass formula derived within AdS/QCD. Consequences for our understanding of QCD are discussed as well as experiments which may help to decide on the validity of models.