Nucleon Resonance Structure from Exclusive Meson Electroproduction with CLAS and CLAS12

The CLAS detector at Jefferson Lab is a unique instrument, which has provided the lionʼs share of the worldʼs data on meson photo- and electroproduction in the resonance excitation region. The electroexcitation amplitudes for the low-lying resonances P33(1232), P11(1440), D13(1520), and S11(1535) were determined over a wide range of Q2<5.0GeV2 in a comprehensive analysis of exclusive single-meson (π+n,π0p) reactions in the electroproduction off protons. Further, CLAS was able to precisely measure π+π−p electroproduction differential cross sections provided by the nearly full kinematic coverage of the detector. The electrocouplings of the P11(1440) and D13(1520) excited states are determined from the exclusive-π+π−p reaction. Consistent results on the electrocouplings from two-independent analyses (single- and double-pion electroproduction) have provided compelling evidence for the reliable extraction of the N⁎ electrocouplings. Preliminary results on the electrocouplings of the S31(1620), S11(1650), D33(1700), and P13(1720) states have recently become available. Theoretical analyses of these results have revealed that there are two major contributions to the resonance structure: a) an internal quark core and b) an external meson-baryon cloud. These CLAS results have had considerable impact on QCD-based studies on N⁎ structure and in the search for manifestations of the dynamical masses of the dressed quarks. Future CLAS12 N⁎ structure studies at high photon virtualities will considerably extend our capabilities in exploring the nature of confinement in baryons.