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

Thomas Jefferson National Accelerator Facility Newport News, Virginia 23606, USA

E-mail: burkert@jlab.org

Published online: 22 September 2020

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 in ternal structure. One focus is on the efforts to obtain a more complete picture of the light-quark baryon exci tation spectrum employing electromagnetic beams that will allow us to draw some conclusions on the symme tries underlying the spectrum. For the higher mass ex citations, the full employment of coupled channel ap proaches 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 de pendences on Q², especially on some of the more promi nent resonances, especially Δ(1232)3/2⁺, N(1440)1/2⁺, and negative parity states N(1535)1/2^-, and N(1675)5/2^-.These were obtained in pion and eta electroproduction experi ments off proton targets and have already led to further insights in the active degrees-of-freedom as a function of the distance scale involved.