N$^*$ Experiments and what they tell us about Strong QCD Physics

TL;DR

This paper reviews experimental studies of nucleon excited states, focusing on spectrum analysis, internal structure, and transition form factors, to enhance understanding of strong QCD and baryon resonances.

Contribution

It provides a comprehensive overview of experimental efforts to map the light-quark baryon spectrum and analyze transition form factors, emphasizing the importance of coupled channel approaches and polarization data.

Findings

Identification of nucleon resonance spectrum features

Insights into internal structure from transition form factors

Evidence supporting specific symmetry patterns in baryon states

Abstract

I give an overview on experimental studies of the spectrum and the structure of the excited states of the nucleon and what we can learn about their internal structure. One focus is on the efforts to obtain a more complete picture of the light-quark baryon excitation spectrum employing electromagnetic beams that will allow us to draw some conclusions on the symmetries underlying the spectrum. For the higher mass excitations, the full employment of coupled channel approaches is essential when searching for new excited states in the large amounts of data already accumulated in different channels involving a variety of polarization observables. The other focus is on the study of transition form factors and helicity amplitudes and their dependences on $Q^2$, especially on some of the more prominent resonances, especially $\Delta(1232)\frac{3}{2}^+$, $N(1440)\frac{1}{2}^+$, and negative parity states $N(1535)\frac{1}{2}^-$, and $N(1675)\frac{5}{2}^-$. These were obtained in pion and eta electroproduction experiments off proton targets and have already led to further insights in the active degrees-of-freedom as a function of the distance scale involved.