Excited nucleon spectrum and structure studies with CLAS and CLAS12

Abstract

The study of the spectrum and structure of excited nucleon states employing the electroproduction of exclusive reactions is an important avenue for exploring the nature of the non-perturbative strong interaction. The CLAS detector in Hall~B has provided the dominant part of the available world data on most relevant meson electroproduction channels off the nucleon in the resonance region for $Q^2$ up to 5~GeV$^2$. Analyses of CLAS data for the exclusive channels $\pi N$, $\eta N$, and $\pi^+ \pi^- p$ on a proton target have provided the only results available on the $Q^2$ evolution of the electro-excitation amplitudes for the transitions from the initial photon-proton to the final $N^*$ states in the mass range up to $W$=1.8~GeV. These electrocouplings allow for exploration of the internal structure of the produced excited nucleon states. This work has made it clear that consistent results from independent analyses of several exclusive channels with different resonance hadronic decay parameters and non-resonant backgrounds but the same $N^*$ electro-excitation amplitudes, is essential to have confidence in the extracted results. Starting in early 2018, a program to study the spectrum and structure of $N^*$ states in various exclusive electroproduction channels using the new CLAS12 spectrometer commenced. These studies will probe the structure of $N^*$ states in the mass range up to $W$=3~GeV and for $Q^2$ as low as 0.05~GeV$^2$ and as high as 10-12~GeV$^2$, thus providing a means to access $N^*$ structure information spanning a broad range of distance scales. Quasi-real photoproduction studies are also planned to search for additional $N^*$ states, the so-called hybrid baryons, for which the glue serves as an active structural component. In this talk the $N^*$ programs from both CLAS and CLAS12 will be reviewed.