Physical Review C
Differential cross sections of the exclusive process ep -> e 'π+n were measured with good precision in the range of the photon virtuality Q2 = 1.8-4.5 GeV2 and the invariant mass range of the π+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 π+center-of-mass system. More than 37 000 cross-section points were measured. The contributions of the isospin I = ½ resonances N(1675) 5/2-, N(1680) 5/2+, and N(1710) 1/2+ were extracted at different values of Q2 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 A3/2 amplitude at the real photon point (Q2 = 0) to a Q2 where A1/2 begins to dominate. The scalar amplitude S 1/2 drops rapidly with Q2consistent with quark model prediction. For the N(1710)½+ resonance our analysis shows significant strength for the A½ amplitude at Q2 < 2.5 GeV2.
Original Publication Citation
The CLAS Collaboration, Park, K., Aznauryan, I. G., Burkert, V. D., Adhikari, K. P., Amaryan, M. J., . . . Zonta, I. (2015). Measurements of ep → e'π+n at 1.6 < w <2.0 GeV and extraction of nucleon resonance electrocouplings at CLAS. Physical Review C, 91(4), 045203. doi:10.1103/PhysRevC.91.045203
The CLAS Collaboration; Park, K.; Adhikari, K. P.; Amaryan, M. J.; Fassi, L. El; Prok, Y.; and Weinstein, L. B., "Measurements of ep → e'π+n at 1.6 < W <2.0 GeV and extraction of nucleon resonance electrocouplings at CLAS" (2015). Physics Faculty Publications. 221.