Baryon spectroscopy

TL;DR

This review discusses the current understanding and challenges in baryon spectroscopy, highlighting the gap between theoretical predictions and experimental observations of baryon states and their properties.

Contribution

It provides a comprehensive overview of baryon spectroscopy, identifying key issues and prospects for future experiments to resolve open questions.

Findings

Quark models explain low-mass excitations well

Mass degeneracy of positive and negative parity states remains unexplained

High-mass resonance density predictions do not match observations

Abstract

About 120 baryons and baryon resonances are known, from the abundant nucleon with $u$ and $d$ light-quark constituents up to the recently discovered $\Omega_b^-=bss$, and the $\Xi_b^-=bsd$ which contains one quark of each generation. In spite of this impressively large number of states, the underlying mechanisms leading to the excitation spectrum are not yet understood. Heavy-quark baryons suffer from a lack of known spin-parities. In the light-quark sector, quark-model calculations have met with considerable success in explaining the low-mass excitations spectrum but some important aspects like the mass degeneracy of positive-parity and negative-parity baryon excitations are not yet satisfactorily understood. At high masses, above 1.8 GeV, quark models predict a very high density of resonances per mass interval which is not observed. In this review, issues are identified discriminating between different views of the resonance spectrum; prospects are discussed how open questions in baryon spectroscopy may find answers from photo- and electro-production experiments which are presently carried out in various laboratories.