Measurements of $ep \to e'\pi^+n$ at 1.6 < W < 2.0 GeV and extraction of nucleon resonance electrocouplings at CLAS

Abstract : 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 $N(1710){1\over2}^+$ resonance our analysis shows significant strength for the $A_{1/2}$ amplitude at $Q^2 < 2.5$ GeV$^2$.
Document type :
Journal articles

http://hal.in2p3.fr/in2p3-01090110
Contributor : Emmanuelle Vernay <>
Submitted on : Wednesday, December 3, 2014 - 8:39:11 AM
Last modification on : Thursday, November 19, 2020 - 12:58:54 PM

Citation

K. Park, I.G. Aznauryan, V.D. Burkert, K.P. Adhikari, M.J. Amaryan, et al.. Measurements of $ep \to e'\pi^+n$ at 1.6 < W < 2.0 GeV and extraction of nucleon resonance electrocouplings at CLAS. Physical Review C, American Physical Society, 2015, 91, pp.045203. ⟨10.1103/PhysRevC.91.045203⟩. ⟨in2p3-01090110⟩

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