Measurements of ep → e'π⁺n at 1.6 < W < 2.0 GeV and extraction of nucleon resonance electrocouplings at CLAS MEASUREMENTS of ep→e′π+n ⋯ K. PARK et al.

Abstract

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 nπ⁺ center-of-mass system. More than 37000 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 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 A₁/₂ 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 S₁/₂ drops rapidly with Q² consistent with quark model prediction. For the N(1710)1/2⁺ resonance our analysis shows significant strength for the A₁/₂ amplitude at Q² < 2.5 GeV².