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.

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=12 resonances N(1675)52-,N(1680)52+, and N(1710)12+ 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)52- 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)52+ 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)12+ resonance our analysis shows significant strength for the A1/2 amplitude at Q2<2.5 GeV2.