Nucleon Resonance Physics

TL;DR

Recent advances in meson photo-production and polarization measurements at electron machines have led to new nucleon and Delta resonance discoveries, revealing their internal structure and supporting theoretical models like lattice QCD.

Contribution

The paper reviews recent experimental discoveries of nucleon resonances and discusses how these findings support and inform theoretical models of baryon structure.

Findings

Discovery of new excited nucleon and Delta states.

Revelation of internal structure of prominent excited states.

Support for lattice QCD predictions of baryon spectrum.

Abstract

Recent results of meson photo-production at the existing electron machines with polarized real photon beams and the measurement of polarization observables of the final state baryons have provided high precision data that led to the discovery of new excited nucleon and $\Delta$ states using multi-channel partial wave analyses procedures. The internal structure of several prominent excited states has been revealed employing meson electroproduction processes. On the theoretical front, lattice QCD is now predicting the baryon spectrum with very similar characteristics as the constituent quark model, and continuum QCD, such as is represented in the Dyson-Schwinger Equations approach and in light front relativistic quark models, describes the non-perturbative behavior of resonance excitations at photon virtuality of $Q^2 > 1.5GeV^2$. In this talk I discuss the need to continue a vigorous program of nucleon spectroscopy and the study of the internal structure of excited states as a way to reveal the effective degrees of freedom underlying the excited states and their dependence on the distance scale probed.