Experimental Challenges of the N* Program

TL;DR

This paper discusses the experimental challenges in exploring the nonperturbative QCD regime through baryon spectroscopy and form factor measurements, emphasizing the need for high-precision data and advanced analysis to understand dressed quark contributions.

Contribution

It highlights the importance of comprehensive measurements and analysis techniques to probe the transition from meson-cloud to dressed-quark dominance in nucleon structure.

Findings

High-precision data on nucleon resonances obtained at JLab

Identification of challenges in extracting electrocoupling parameters

Progress towards QCD-based understanding of nucleon structure

Abstract

The first challenge faced in investigating the strong interaction from partially explored, where meson-cloud degrees of freedom dominate, to still unexplored distance scales, where the dressed-quark contributions are the dominating degrees of freedom, is to find an experiment that allows to measure observables that are probing this evolving nonperturbative QCD regime over the full range. Baryon spectroscopy can establish more sensitively, and in an almost model-independent way, nucleon excitation and non-resonant reaction amplitudes by complete measurements of pseudo-scalar meson photoproduction off nucleons. Elastic and transition form factors can then trace this evolution by measurements of elastic electron scattering and exclusive single-meson and double-pion electroproduction cross sections off the nucleon that will be extended to higher momentum transfers with the energy-upgraded CEBAF beam at JLab to study the dressed 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 immanent next challenge is a high-quality analysis to extract these relevant electrocoupling parameters for various resonances that then can be compared to state of the art models and QCD-based calculations. Recent results demonstrate the status of the analysis and pinpoint further challenges, including those to establish QCD-based results directly from the experimental data.