Volker Burkert
Jefferson Laboratory

Ralf Gothe
University of South Carolina

Craig Roberts
Argonne National Laboratory

Adam Szczepaniak
Indiana University

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Farha Habib
(206) 685-4286

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INT Workshop INT-16-62W
Spectrum and Structure of Excited Nucleons from Exclusive Electroproduction

November 14 - 18, 2016

The international program aimed at determining the spectrum and structure of nucleon-resonances provides a unique tool with which to forge an understanding of essentially nonperturbative features of the Standard Model. It will provide input that is crucial to answering three questions that lie at the heart of hadron physics: what is confinement; what is dynamical chiral symmetry breaking (DCSB); and how are they related? In this connection, the last decade has witnessed enormous growth in the body of data obtained via meson photo- and electro-production from nucleons. That data has vastly improved our knowledge of the baryon spectrum, establishing the existence of new baryon states, and enabled empirical determination of the resonance electrocouplings, gvNN* (Q2), of most nucleons with mass less-than 2 GeV at photon virtualities up-to 5 GeV2. Recently, analyses of the data and feedback between experiment and theory have begun to reveal the full potential of the program. For example, we have seen: a successful description of nucleon elastic form factors, nucleon-to-Δ(1232)3/2+ and nucleon-to-N(1440)1/2+ transition form factors using QCD's Dyson-Schwinger equations (DSEs) in a unified treatment that suggests the dressed-quark mass-function, M(k), is insensitive to the hadronic environment and hence readily accessible to experiment; a combination of lattice-QCD and light-cone sum rules providing parton distribution amplitudes for ground- and excited-state nucleons and therefrom a first-principles explanation of N(1535)1/2- electrocouplings; and a growing appreciation, from the combined used of dynamical coupled channels models, DSEs and advanced quark models, that the properties of excited nucleons should be seen as arising from a complex interplay between a long-range meson-baryon cloud and a core of three dressed-quarks, which is directly accessible at high photon virtualities.

Into the future, following completion of the JLab 12 GeV upgrade, CLAS12 will be capable of determining the electrocouplings, gvNN* (Q2), of almost all prominent N* states at unprecedented photon virtualities: Q2 ∈ [6, 12] GeV2, providing access to the dressed-quark cores of a diverse array of baryons and delivering empirical information that is necessary in order to address a wide range of critical issues throughout the domain upon which QCD dressing of quarks shifts from being essentially nonperturbative to perturbative in character, a transition that is anticipated to have a material impact on observables, and whose nature and manifestations have been long been the subject of intense interest and debate. Complementing these features, the CLAS12 program will also be able to extend the Q2-coverage of electrocouplings down to values of 0.01 GeV2, thereby providing a promising and novel tool in the search for new states of baryon matter, the "hybrid baryons", with a structural (or valence) glue component.

A synergistic relationship between experimental and theoretical efforts has already delivered novel insights into the spectrum and structure of baryons. An expansion of this effort is crucial to success as the Community works toward complete elucidation of the strong interaction sector of the Standard Model, which includes an explanation of the origin, nature and number of nucleon excited-states that emerge from QCD. This Workshop will serve that purpose, and create opportunities and foster growth in a field that is central to the mission of nuclear and hadron physics. Its foci and themes will include:

  • Updates on the N* spectrum from exclusive photoproduction off nucleons

  • N* spectrum/structure from exclusive meson electroproduction: new prospects with the CLAS12

  • Amplitude analysis and reaction models for extraction of resonance electrocouplings

  • Resonance electrocouplings as a tool for advancing knowledge of parton distributions in the nucleon and its excited states

  • N* spectrum and structure from DSE- and lattice-QCD, and from quark models modernized using wellmotivated QCD-input

  • Searches for new and hybrid baryons

There will be a $45 registration fee to attend this workshop. The registration fee includes participation in the workshop, lectures, and coffee breaks.