Progress Toward Understanding Baryon Resonances 2
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REVIEW ARTICLE Progress Toward Understanding Baryon Resonances V. Crede and W. Roberts Florida State University, Department of Physics, Tallahassee, FL 32306, USA E-mail: [email protected], [email protected] 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. PACS numbers: 12.39.-x Phenomenological quark models, 13.60.Le Meson production, 13.60.-r Photon and charged-lepton interactions with hadrons, 13.75.Gx Pion-baryon interactions, 25.20.Lj Photoproduction reactions arXiv:1302.7299v2 [nucl-ex] 24 May 2013 Progress Toward Understanding Baryon Resonances 2 1. Introduction A better understanding of the nucleon as a bound state of quarks and gluons as well as the spectrum and internal structure of excited baryons remains a fundamental challenge and goal in hadronic physics. In particular, the mapping of the nucleon excitations provides access to strong interactions in the domain of quark confinement. While the peculiar phenomenon of confinement is experimentally well established and believed to be true, it remains analytically unproven and the connection to quantum chromodynamics (QCD) – the fundamental theory of the strong interactions – is only poorly understood. In the early years of the 20th century, the study of the hydrogen spectrum has established without question that the understanding of the structure of a bound state and of its excitation spectrum need to be addressed simultaneously. The spectroscopy of excited baryon resonances and the study of their properties is thus complementary to understanding the structure of the nucleon in deep inelastic scattering experiments that provide access to the properties of its constituents in the ground state. However, the collective degrees of freedom in such experiments are lost. An extensive data set of observables in light-meson photo- and electro-production reactions has been accumulated over recent years at facilities worldwide such as Jefferson Laboratory in the United States, the ELectron Stretcher Accelerator (ELSA), the MAinz MIcrotron (MAMI), and the GRenoble Anneau Accelerateur Laser (GRAAL) facility in Europe as well as the 8 GeV Super Photon Ring (SPring-8) in Japan hosting the Laser Electron Photon Experiment (LEPS). The data set includes cross section data and polarisation observables for a large variety of final states, such as πN, ηN, ωN, ππN, KΛ, KΣ, etc. These data complement the earlier spectroscopy results from π- and K-induced reactions. For a long time, the lack of experimental data and the broad and overlapping nature of light-flavour baryon resonances has obscured our view of the nucleon excitation spectrum. These experiments therefore represent an important step toward the unambiguous extraction of the scattering amplitude in these reactions, which will allow the identification of individual resonance contributions. Hadronic spectroscopy cannot be interpreted by applying standard perturbation theory: phenomenology as well as QCD-based models have provided much of the insight and theoretical guidance. The recent advances in lattice gauge theory and the availability of large-scale computing technology make it possible for the first time to complement these approaches with numerical solutions of QCD. Spin-parity assignments for excited states have even been successfully worked out by some groups. Some collaborations have carried out simulations with pion masses as light as 156 MeV, but the resonance nature of the states, as well as the presence of thresholds complicate the extraction of information from such calculations. In addition, to the best of our knowledge, such simulations have focused on states with lower values of angular momentum. In other simulations with larger pion masses (mπ & 400 MeV), a rich spectrum of excited states is obtained, and the low-lying states of some lattice-QCD calculations have the same quantum numbers as the states in models based on three Progress Toward Understanding Baryon Resonances 3 quarks with wave functions based on the irreducible representations of SU(6) O(3). ⊗ The good qualitative agreement may be surprising since the connection between the relevant quark degrees of freedom, the constituent or dressed quarks, and those of the QCD Lagrangian is not well understood. The lattice results appear to answer the long- standing question in hadron spectroscopy of whether the large number of excited baryons predicted by quark models, but experimentally not observed, is realised in nature. The main goals of recent experiments are the determination of the excited baryon spectrum, the identification of possible new symmetries in the spectrum, and understanding the structure of states that appear to be built of three valence quarks at a microscopic level. The fundamental questions comprise the quest for the number of relevant degrees of freedom and a better understanding of the mechanisms responsible for confinement and chiral symmetry breaking: How are the valence or dressed quarks with their clouds of gluons and quark-antiquark pairs related to the quark and gluon fields of the underlying Lagrangian of QCD? How does the chiral symmetry structure of QCD lead to dressed quarks and produce the well-known long distance behaviour? The new experimental and theoretical information bears directly on both the search for, and our current understanding of baryon resonances. This paper will review the new data, present an overview of theoretical approaches and a sampling of the phenomenological work that has been developed based on the new experimental results. 1.1. Guide to the Literature The Particle Data Group (PDG) regularly includes minireviews on a large variety of topics in their Reviews of Particle Properties (RPP), which have been published biennially for many decades. Useful minireviews on N ∗ and ∆ resonances as well as charmed baryons can be found in the latest edition of the RPP [1]. Owing to the lack of suitable K beams, little progress has been reported in reviews on Λ and Σ resonances in the 2010 edition of the RPP [2]. The latter also includes a very brief note on Ξ baryons based on a previous review by Meadows, published in the proceedings of the Baryon 1980 conference [3]. A comprehensive review of baryon spectroscopy is contained in the 2009 article by Klempt and Richard [4], who discuss prospects for photo- and electroproduction experiments. Some of the open questions discussed in that review have already found answers from recent measurements. An older general article on baryon spectroscopy by Hey and Kelly [5] still provides useful information, particularly on some aspects of the theoretical data analysis. Quark model developments have been discussed by Capstick and Roberts in 2000 [6]. Further reviews were dedicated to particular aspects of experiments using electromagnetic probes. About a decade ago, Krusche and Schadmand gave a nice summary on low-energy photoproduction [7] and in 2007, Drechsel and Walcher looked at hadron structure at low Q2 [8]. The work of Tiator et al. [9] and Aznauryan and Burkert [10] provide more recent reviews on the electroexcitation of nucleon resonances. Progress Toward Understanding Baryon Resonances 4 2. Baryon Spectroscopy Baryons are strongly interacting fermions and all established baryons are consistent with 1 1 1 a qqq configuration, so that the baryon number is B = 3 + 3 + 3 = 1. Other exotic baryons have been proposed, such as pentaquarks – baryons made of four quarks and one antiquark (B = 1 + 1 + 1 + 1 1 = 1), but their existence is not generally accepted. In 3 3 3 3 − 3 quantum chromodynamics, heptaquarks (5 quarks, 2 antiquarks), nonaquarks (6 quarks, 3 antiquarks), etc. could also exist. Baryons which consist only of u and d quarks are 1 3 called nucleons if they have isospin I = 2 or ∆ resonances if they have isospin I = 2 . Baryons containing s quarks are called hyperons and are labelled as Λ, Σ, Ξ, and Ω depending on the number of s quarks and isospin (Table 1). Baryons containing c or + b quarks are labelled with an index. For example, the Λc has isospin zero and quark + ++ content udc, the Ξc has quark content usc and the Ξcc has quark content ucc. 2.1. Baryons Composed of u, d and s Quarks As fermions baryons obey the Pauli principle, so the total wave function qqq = colour space, spin, flavour (2.1) | iA | iA × | iS must be antisymmetric (denoted by the index A) under the interchange of any two equal- mass quarks. Since all observed hadrons are colour singlets, the colour component of the wave function must be completely antisymmetric. For the light-flavour baryons, the three flavors u, d and s can be treated in an approximate SU(3) framework, in which each quark is a member of an SU(3) triplet. The flavour wave functions of baryon states can then be constructed to be members of SU(3) multiplets as 3 3 3 = 10 8 8 1 .