| We present results on the electroexcitation of the low mass resonances Delta(1232)P33, N(1440)P11, N(1520)D13, and N(1535)S11 in a wide range of Q2. The results were obtained in the comprehensive analysis of JLab-CLAS data on differential cross sections, longitudinally polarized beam asymmetries, and longitudinal target and beam-target asymmetries for pion electroproduction off the proton. The data were analysed 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 Delta(1232)P33 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 Q2<6GeV2. For the Roper resonance, N(1440)P11, the data provide strong evidence for this state as a predominantly radial excitation of a 3-quark ground state. Measured in pion electroproduction,
the transverse helicity amplitude for the N(1535)S11 allowed us to obtain the branching ratios of this state to the pi-N and eta-N channels via comparison to the results extracted from eta electroproduction. The precise CLAS measurements also allowed to extract the longitudinal transition amplitudes for the N(1535)S11 and N(1520)D13 states with good precision. For the N(1535)S11, these results became a challenge for quark models, and may be indicative of large meson-cloud contributions or of representations of this state different from a 3q excitation. The transverse amplitudes for the N(1520)D13 clearly show the rapid changeover from helicity-3/2 dominance at the real photon point to helicity-1/2 dominance at Q2>1GeV2, confirming a long-standing prediction of the constituent quark model. |
