Measurements ofep→e′π+nat1.6<W<2.0 GeV and extraction of nucleon resonance electrocouplings at CLAS

Differential cross sections of the exclusive process $$e p \to e^\prime \pi^+ n$$ were measured with good precision in the range of the photon virtuality $Q^2 = 1.8 - 4.5$ GeV$^2$, and the invariant mass range of the $$\pi^+ n$$ final state W = 1.6 - 2.0 GeV using the CEBAF 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 $$I = {1\over 2}$$ resonances $$N(1675){5\over 2}^-$$, $$N(1680){5\over 2}^+$$ and $$N(1710){1\over 2}^+$$ 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 $$N(1675){5\over 2}^-$$ 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 $$N(1680){5\over 2}^+$$ 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 $$N(1710){1\over 2}^+$$ resonance our analysis shows significant strength for the $$A_{1/2}$$ amplitude at $Q^2 < 2.5$ GeV$^2$.