Electroexcitation of nucleon resonances

Abstract

We review recent progress in the investigation of the electroexcitation of nucleon resonances, both in experiment and in theory. We describe current experimental facilities, the experiments performed on π and η electroproduction off protons, and theoretical approaches used for the extraction of resonance contributions from the experimental data. The status of 2 π , K Λ , and K Σ electroproduction is also presented. The most accurate results have been obtained for the electroexcitation amplitudes of the four lowest excited states, which have been measured in a range of Q 2 up to 8 and 4.5 GeV 2 for Δ ( 1232 ) P 33 , N ( 1535 ) S 11 and N ( 1440 ) P 11 , N ( 1520 ) D 13 , respectively. These results have been confronted with calculations based on lattice QCD, large- N c relations, perturbative QCD (pQCD), and QCD-inspired models. The amplitudes for the Δ ( 1232 ) indicate large pion-cloud contributions at low Q 2 and do not show any sign of approaching the pQCD regime for Q 2 < 7 GeV 2 . Measured for the first time, the electroexcitation amplitudes of the Roper resonance, N ( 1440 ) P 11 , provide strong evidence for this state as a predominantly radial excitation of a three-quark (3 q ) ground state, with additional non-3-quark contributions needed to describe the low Q 2 behavior of the amplitudes. The longitudinal transition amplitude for the N ( 1535 ) S 11 was determined and has become a challenge for quark models. Explanations may require large meson-cloud contributions or alternative representations of this state. The N ( 1520 ) D 13 clearly shows the rapid changeover from helicity-3/2 dominance at the real photon point to helicity-1/2 dominance at Q 2 > 0.5 GeV 2 , confirming a long-standing prediction of the constituent quark model. The interpretation of the moments of resonance transition form factors in terms of transition transverse charge distributions in infinite momentum frame is presented.