Electroexcitation of nucleon resonances from CLAS data on single pion electroproduction

Abstract

We present results on the electroexcitation of the low mass resonances Δ(1232)P[subscript 33], N(1440)P[subscript 11], N(1520)D[subscript 13], and N(1535)S11 in a wide range of Q[superscript 2]. The results were obtained in the comprehensive analysis of data from the Continuous Electron Beam Accelerator Facility (CEBAF) large acceptance spectrometer (CLAS) detector at the Thomas Jefferson National Accelerator Facility (JLab) on differential cross sections, longitudinally polarized beam asymmetries, and longitudinal target and beam-target asymmetries for π electroproduction off the proton. The data were analyzed using two conceptually different approaches—fixed-t dispersion relations and a unitary isobar model—allowing us to draw conclusions on the model sensitivity of the obtained electrocoupling amplitudes. The amplitudes for the Δ(1232)P[subscript 33] show the importance of a meson-cloud contribution to quantitatively explain the magnetic dipole strength, as well as the electric and scalar quadrupole transitions. They do not show any tendency of approaching the pQCD regime for Q[superscript 2] ≦ 6 GeV2. For the Roper resonance, N(1440)P[subscript 11], the data provide strong evidence that this state is a predominantly radial excitation of a three-quark (3q) ground state. Measured in pion electroproduction, the transverse helicity amplitude for the N(1535)S[subscript 11] allowed us to obtain the branching ratios of this state to the πN and ηN channels via comparison with the results extracted from η electroproduction. The extensive CLAS data also enabled the extraction of the γ ∗ p → N(1520)D[subscript 13] and N(1535)S[subscript 11] longitudinal helicity amplitudes with good precision. For the N(1535)S[subscript 11], these results became a challenge for quark models and may be indicative of large meson-cloud contributions or of representations of this state that differ from a 3q excitation. The transverse amplitudes for the N(1520)D[subscript 13] clearly show the rapid changeover from helicity-3/2 dominance at the real photon point to helicity-1/2 dominance at Q2 > 1 GeV[superscript 2], confirming a long-standing prediction of the constituent quark model.