Measurements of $ep\to e^{\prime }{\pi }^{+}n$ at W = 1.6 - 2.0 GeV and extraction of nucleon resonance electrocouplings at CLAS

Abstract

Differential cross sections of the exclusive process $ep\to e^{\prime }{\pi }^{+}n$ 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 $I=\frac{1}{2}$ resonances $N(1675){\frac{5}{2}}^{-}$, $N(1680){\frac{5}{2}}^{+}$ and $N(1710){\frac{1}{2}}^{+}$ 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 $N(1675){\frac{5}{2}}^{-}$ 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 $N(1680){\frac{5}{2}}^{+}$ 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 $N(1710){\frac{1}{2}}^{+}$ resonance our analysis shows significant strength for the $A_{1/2}$ amplitude at $Q^2<2.5$ GeV${}^2$.

24 pages, 22 figures. arXiv admin note: substantial text overlap with arXiv:0709.1946

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