We present results on the electroexcitation of the low mass resonances (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 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 (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 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 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 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.

The atomic nucleus is composed of two different kinds of fermions: protons and neutrons. If the protons and neutrons did not interact, the Pauli exclusion principle would force the majority of fermions (usually neutrons) to have a higher average momentum. Our high-energy electron-scattering measurements using (12)C, (27)Al, (56)Fe, and (208)Pb targets show that even in heavy, neutron-rich nuclei, short-range interactions between the fermions form correlated high-momentum neutron-proton pairs. Thus, in neutron-rich nuclei, protons have a greater probability than neutrons to have momentum greater than the Fermi momentum. This finding has implications ranging from nuclear few-body systems to neutron stars and may also be observable experimentally in two-spin-state, ultracold atomic gas systems.

A study of the P 11 (1440) The transition helicity amplitudes from the proton ground state to the P11(1440) and D13(1520) excited states (γvpN * electrocouplings) were determined from the analysis of nine independent onefold differential π + π − p electroproduction cross sections off a proton target, taken with CLAS at photon virtualities 0.25 GeV 2 < Q 2 < 0.60 GeV 2 . The phenomenological reaction model was employed for separation of the resonant and non-resonant contributions to the final state. The P11(1440) and D13(1520) electrocouplings were obtained from the resonant amplitudes parametrized within the framework of a unitarized Breit-Wigner ansatz. They are consistent with results obtained in the previous CLAS analyses of the π + n and π 0 p channels. The successful description of a large body of data in dominant meson-electroproduction channels off protons with the same γvpN * electrocouplings offers clear evidence for the reliable extraction of these fundamental quantities from meson-electroproduction data. This analysis also led to the determination of the long-awaited hadronic branching ratios for the D13(1520) decay into ∆π (24%-32%) and N ρ (8%-17%).

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High-statistics differential cross sections for the reactions γp → pη and γp → pη have been measured using the CEBAF large acceptance spectrometer (CLAS) at Jefferson Lab for center-of-mass energies from near threshold up to 2.84 GeV. The η results are the most precise to date and provide the largest energy and angular coverage. The η measurements extend the energy range of the world's large-angle results by approximately 300 MeV. These new data, in particular the η measurements, are likely to help constrain the analyses being performed to search for new baryon resonance states.