Progress towards understanding baryon resonances

Abstract

Abstract. The composite nature of baryons manifests itself in the existence of a rich spectrum of excited states, in particular in the important mass region 1-2 GeV for the light-flavoured baryons. The properties of these resonances can be identified by systematic investigations using electromagnetic and strong probes, primarily with beams of electrons, photons, and pions. After decades of research, the fundamental degrees of freedom underlying the baryon excitation spectrum are still poorly understood. The search for hitherto undiscovered but predicted resonances continues at many laboratories around the world. Recent results from photo-and electroproduction experiments provide intriguing indications for new states and shed light on the structure of some of the known nucleon excitations. The continuing study of available data sets with consideration of new observables and improved analysis tools have also called into question some of the earlier findings in baryon spectroscopy. Other breakthrough measurements have been performed in the heavy-baryon sector, which has seen a fruitful period in recent years, in particular at the B factories and the Tevatron. First results from the LHC indicate rapid progress in the field of bottom baryons. In this review, we discuss the recent experimental progress and give an overview of theoretical approaches.

Baryons as relativistic three-quark bound states

“…In recent years, a large amount of data on photoproduction on several final states (γN → πN, ηN, ππN , etc.) has been accumulated at ELSA/Bonn, GRAAL/Grenoble, MAMI/Mainz, Jefferson Lab and Spring-8 in Osaka; see [4,10] for reviews. The data set obtained by measurements of high-precision cross sections and polarisation observables in pion photoproduction is coming close towards a complete experiment, where one is able to extract the four complex amplitudes involved in the process unambiguously, and combining precision data with the development of multichannel partial-wave analyses has led to the addition of several new states to the PDG.…”

“…Currently, both photo-and electroproduction of mesons off nucleons constitute the main experimental approaches for the discovery of new nucleon resonances and our understanding of electromagnetic baryon structure in the spacelike momentum region. In the following we will give a very brief survey of the most prominent resonances and their basic properties; more details can be found in the dedicated reviews [4,[10][11][12][13][14].…”

“…If we introduce a variable δ by writing s = m 2 (1 + 4δ), then their relations to the Lorentz invariants s and τ are given by 10) with µ = m 2 π /(4m 2 ). The CM scattering angle defined by q · k = |q||k| cos θ additionally depends on t and it is related to the Lorentz invariants via…”

“…The UV term ensures the correct perturbative running but is otherwise not essential; one could neglect it without causing serious damage in the light-quark sector [352]. The nonperturbative physics is encoded in the first term with x = k 2 /Λ 2 , which is characterised by two parameters 10 : an infrared scale Λ and a dimensionless parameter η. Since the scale Λ (together with the quark masses) is fixed to experimental input, only one free parameter η remains to which many observables are insensitive within the range plotted in Fig.…”

Production of η and η′ mesons on nucleons and nuclei

“…The main motivation for the study of photon-induced meson production off nucleons is the investigation of the excitation spectrum of the nucleon, which is one of the most important testing grounds for Quantum Chromodynamics (QCD) in the non-perturbative regime (see [21] for a recent summary). In the past, most information about excited nucleon states was gained from pion scattering; even today most states listed in the summary tables of the Review of Particle Physics (PDG) [22] were 'discovered' in this reaction.…”