Studies of Nucleon Resonance Structure in Exclusive Meson Electroproduction

Abstract

Studies of the structure of excited baryons are key factors to the N* program at Jefferson Lab (JLab). Within the first year of data taking with the Hall B CLAS12 detector following the 12 GeV upgrade, a dedicated experiment will aim to extract the N* electrocouplings at high photon virtualities Q2. This experiment will allow exploration of the structure of N* resonances at the highest photon virtualities ever achieved, with a kinematic reach up to Q2 = 12 GeV 2. This high-Q2 reach will make it possible to probe the excited nucleon structures at distance scales ranging from where effective degrees of freedom, such as constituent quarks, are dominant through the transition to where nearly massless bare-quark degrees of freedom are relevant. In this document, we present a detailed description of the physics that can be addressed through N* structure studies in exclusive meson electroproduction. The discussion includes recent advances in reaction theory for extracting N* electrocouplings from meson electroproduction off protons, along with Quantum Chromodynamics (QCD)-based approaches to the theoretical interpretation of these fundamental quantities. This program will afford access to the dynamics of the nonperturbative strong interaction responsible for resonance formation, and will be crucial in understanding the nature of confinement and dynamical chiral symmetry breaking in baryons, and how excited nucleons emerge from QCD.

Baryons as relativistic three-quark bound states

“…Naturally, our own bias is towards the Dyson-Schwinger/Bethe-Salpeter approach and therefore we will mainly concentrate on pointing out its connections with other methods and the inherent similarities and differences. We apologize to those colleagues whose work is not mentioned here; for more comprehensive discussions we refer to the existing reviews focusing on nucleon spacelike electromagnetic form factors [440][441][442][443][444][445][446], axial form factors [447,448], transition form factors [11][12][13][14], and timelike form factors [445,449].…”

“…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].…”

“….} in the Lagrangian 14) where the averaged fieldΦ is the expectation value of Φ in the presence of sources J. The effective action can be expanded in the 1PI vertex functions V (N ) i 1 ,...,i N and, vice versa, they can be derived from it via functional derivatives…”

“…It depends on 10 functions which must be known beforehand: the quark dressing functions A(k 2 ) and M (k 2 ) and the eight transverse form factors f i (k 2 , z 2 , Q 2 ) of the offshell quark-photon vertex. 14 In Fig. 4.6 we show the Dyson-Schwinger result for the Adler function D(Q 2 ) = −Q 2 dΠ(Q 2 )/dQ 2 obtained by calculating the quark propagator and quark-photon vertex consistently in a rainbow-ladder truncation [438].…”

Modern Dynamical Coupled-Channels Calculations for Extracting and Understanding the Nucleon Spectrum

“…Because of this, the electromagnetic transition form factors are being actively investigated both experimentally and theoretically, and this has opened a great opportunity to make a quantitative study of the substructure of the N * and ∆ * resonances in close relation with experimental data (see, e.g., Ref. [18]). The right side of Fig.…”