Baryon spectroscopy

About 120 baryons and baryon resonances are known, from the abundant nucleon with $u$ and $d$ light-quark constituents up to the ${\ensuremath{\Xi}}_{b}^{\ensuremath{-}}=(bsd)$, which contains one quark of each generation and to the recently discovered ${\ensuremath{\Omega}}_{b}^{\ensuremath{-}}=(bss)$. 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 such as the mass degeneracy of positive-parity and negative-parity baryon excitations remain unclear. At high masses, above $1.8\phantom{\rule{0.3em}{0ex}}\mathrm{GeV}$, quark models predict a very high density of resonances per mass interval which is not yet observed. In this review, issues are identified discriminating between different views of the resonance spectrum; prospects are discussed on how open questions in baryon spectroscopy may find answers from photoproduction and electroproduction experiments which are presently carried out in various laboratories.

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