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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.


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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