Impact of the $\gamma _{\nu }$NN* Electrocoupling Parameters at High Photon Virtualities and Preliminary Cross Sections off the Neutron

Abstract

Meson-photoproduction measurements and their reaction-amplitude analyses can establish more sensitively, and in some cases in an almost model-independent way, nucleon excitations and non-resonant reaction amplitudes. However, to investigate the strong interaction from already explored—where meson-cloud degrees of freedom contribute substantially to the baryon structure—to still unexplored distance scales—where quark degrees of freedom dominate and the transition from dressed to current quarks occurs—we depend on experiments that allow us to measure observables that are probing this evolving non-perturbative QCD regime over its full range. Elastic and transition form factors are uniquely suited to trace this evolution by measuring elastic electron scattering and exclusive single-meson and double-pion electroproduction cross sections off the nucleon. These exclusive measurements will be extended to higher momentum transfers with the energy-upgraded CEBAF beam at JLab to study the quark degrees of freedom, where their strong interaction is responsible for the ground and excited nucleon state formations. After establishing unprecedented high-precision data, the imminent next challenge is a high-quality analysis to extract these relevant electrocoupling parameters for various resonances that can then be compared to state-of-the-art models and QCD-based calculations. The vast majority of the available exclusive electroproduction cross sections are off the proton. Hence flavor-dependent analyses of excited light-quark baryons are lacking experimental data off the neutron. The goal is to close this gap by providing exclusive ${\gamma }_{\nu }(n) \rightarrow p^{+} {\pi }^{-}$ reaction cross section off deuterium and to establish a kinematical final-state-interaction (FSI) correction factor (R) map that can be determined from the data set itself. The “e1e” Jefferson Lab CLAS data set, that is analyzed, includes both a hydrogen and deuterium target run period, which allows a combined analysis of the pion electroproduction off the free proton, the bound proton, and the bound neutron under the same experimental conditions. Hence it will provide the experimentally best possible information on the off-shell and FSI effects in deuterium, which must be considered in order to extract the information off the neutron. The cross section analysis of this data set, that is currently underway, will considerably improve our knowledge of the $Q^{2}$ evolution of resonance states off bound protons and neutrons. Recent results presented here and in these proceedings are demonstrating the status and continuous progress of data analyses and their theoretical descriptions, as well as highlighting the experimental and theoretical outlook of what shall and may be achieved in the new era of the 12-GeV upgraded transition form factor program.