Physical Review C
Differential cross sections of the exclusive process ep -> e 'pi(+)n were measured with good precision in the range of the photon virtuality Q(2) = 1.8-4.5 GeV2 and the invariant mass range of the pi(+)n final state W = 1.6-2.0 GeV using the Continuous Electron Beam Accelerator Facility Large Acceptance Spectrometer. Data were collected with nearly complete coverage in the azimuthal and polar angles of the n pi(+) center-of-mass system. More than 37 000 cross-section points were measured. The contributions of the isospin I = 1/2 resonances N(1675) 5/2(-), N(1680) 5/2(+), and N(1710) 1/2(+) were extracted at different values of Q(2) using a single-channel, energy-dependent resonance amplitude analysis. Two different approaches, the unitary isobar model and the fixed-t dispersion relations, were employed in the analysis. We observe significant strength of the N(1675)5/2(-) in the A(1/2) amplitude, which is in strong disagreement with quark models that predict both transverse amplitudes to be strongly suppressed. For the N(1680)5/2(+) we observe a slow changeover from the dominance of the A(3/2) amplitude at the real photon point (Q(2) = 0) to a Q(2) where A(1/2) begins to dominate. The scalar amplitude S-1/2 drops rapidly with Q(2) consistent with quark model prediction. For the N(1710)1/2(+) resonance our analysis shows significant strength for the A1/2 amplitude at Q(2) < 2.5 GeV2.
Park, K., Aznauryan, I. G., Burkert, V. D., Adhikari, K. P., Amaryan, M. J., Pereira, S. A., ... & Biselli, A. S. (2015). Measurements of e p→ e′ π+ n at 1.6< W< 2.0 GeV and extraction of nucleon resonance electrocouplings at CLAS. Physical Review C, 91(4), 045203.