Electroexcitation of nucleon resonances from CLAS data on single pion electroproduction

We present results on the electroexcitation of the low mass resonances $\ensuremath{\Delta}(1232){P}_{33}$, $N(1440){P}_{11}$, $N(1520){D}_{13}$, and $N(1535){S}_{11}$ in a wide range of ${Q}^{2}$. 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 $\ensuremath{\pi}$ 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 $\ensuremath{\Delta}(1232){P}_{33}$ 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 ${Q}^{2}\ensuremath{\leqslant}6$ GeV${}^{2}$. For the Roper resonance, $N(1440){P}_{11}$, 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 $N(1535){S}_{11}$ allowed us to obtain the branching ratios of this state to the $\ensuremath{\pi}N$ and $\ensuremath{\eta}N$ channels via comparison with the results extracted from $\ensuremath{\eta}$ electroproduction. The extensive CLAS data also enabled the extraction of the ${\ensuremath{\gamma}}^{*}p\ensuremath{\rightarrow}N(1520){D}_{13}$ and $N(1535){S}_{11}$ longitudinal helicity amplitudes with good precision. For the $N(1535){S}_{11}$, 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 $N(1520){D}_{13}$ clearly show the rapid changeover from helicity-3/2 dominance at the real photon point to helicity-1/2 dominance at ${Q}^{2}>1$ GeV${}^{2}$, confirming a long-standing prediction of the constituent quark model.