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Spectrum and Structure of Excited Nucleons from Exclusive Electroproduction (INT-16-62W)
(INT Workshop 14-18 November 2016)

Reported by V. D. Burkert, R. W. Gothe, V. Mokeev, C. D. Roberts, and A. Szczepaniak
  Date posted May 4, 2017

Two of the most basic goals of nuclear physics are to understand the mechanisms responsible for the formation of hadrons and explain their structure in terms of the gluons and quarks of quantum chromodynamics (QCD). In this connection, measurements of the nucleon excitation (N*) spectrum and exploration of N* structure through the study of resonance electro-excitation amplitudes (γνNN* electrocouplings) on a broad domain of photon virtualities (Q2) present unique opportunities for elucidating these crucial aspects of nonperturbative strong-interaction dynamics. The complexity of this problem is great, so a multipronged approach is necessary to achieve its solution.


    The nucleon resonance spectrum and structure from exclusive meson photo- and electroproduction data. (Left) Spectrum of excited nucleon states as of 2017. Information on new baryon states (highlighted by green boxes) emphasizes the major impact of recent CLAS KY photoproduction data. (Right) Consistent results on N(1440)1/2+ electrocouplings from CLAS Nπ and π+πp electroproduction data. The description of this data provided by both DSE computations, with a traceable connection to QCD, and light-front quark models, defined with momentum-dependent quark masses, highlights that such data provide access to the mechanisms in strong-QCD that underlie hadron mass generation and reveals that N* structure emerges from a complex interplay between an inner core of dressed quarks and an external meson cloud.

This workshop built upon the success of earlier meetings - at JLab (2008, 2011), U. South Carolina (2012), and ECT* in Trento (2015) - with a continued focus on facilitating interactions between expert practitioners representing the various, complementary approaches to this challenging problem. The workshop gathered more than 30 participants, and explored themes ranging from basic theory through practical phenomenology and onto the means by which conjectures and predictions can be verified empirically via studies of exclusive electroproduction. The range and impact of this program will be greatly expanded by experiments enabled by the CLAS12 detector in the new JLab12 era.

An important achievement of this workshop was agreement on a "Roadmap for the Future", which includes

  • Developing a synergistic experiment-theory effort aimed at charting and understanding the Q2-dependence of N* structure, with particular emphasis on the transition from infrared momenta, where meson-baryon final-state-interactions (MB-FSIs) are important, to the domain of dominance by the dressed-quark core.

  • Continued refinement of those theoretical approaches capable of relating experimental results on the N* spectrum, electroexcitations, and hadronic decay amplitudes to strong QCD dynamics, including both improved truncation schemes for continuum-QCD theoretical tools and sound phenomenological models; and the pursuit of a lattice-QCD extraction of the N* spectrum and structure.

  • Building reaction models that respect general constraints on amplitudes, such as unitarity, analyticity and crossing symmetry, for the extraction of resonance properties from experimental data or lattice-QCD simulations at finite volume; and models that explicitly and meaningfully incorporate quark degrees-of-freedom in the description of the non-resonant components of electroproduction amplitudes.

  • Developing methods required for identification of hybrid baryon states in analyses of data on Nπ, Nππ, and KY exclusive electroproduction off protons using the CLAS12 detector, with particular emphasis on those techniques which can distinguish between bound quark and open channel MB-FSI effects.

CLAS12 is the only facility foreseen worldwide that will be able to search for hybrid baryons in electroproduction processes and, in providing access to an enormous range of final-state baryon quantum numbers with electroproduction transitions at up to Q2~12GeV2, it is uniquely capable of exposing many facets of the transition between the non-perturbative and perturbative domains of QCD. It follows that the associated experimental and theoretical programs, working together, will supply crucial insights concerning some of the most pressing questions in modern science, viz. what is the origin of more than 98% of the visible mass, how is it connected with confinement, and how both phenomena emerge from the Standard Model.