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Title: Measurements of $$ep \to e^\prime π^+n$$ at W = 1.6 - 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$^2$.
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Publication Date:
OSTI Identifier:
1178732
Report Number(s):
JLAB-PHY--15-4; DOE/OR/23177--3253; arXiv:1412.0274
Journal ID: ISSN 0556-2813; PRVCAN; ArticleNumber: 045203; TRN: US1600817
Grant/Contract Number:
AC05-06OR23177
Type:
Accepted Manuscript
Journal Name:
Physical Review. C, Nuclear Physics
Additional Journal Information:
Journal Volume: 91; Journal Issue: 4; Journal ID: ISSN 0556-2813
Publisher:
American Physical Society (APS)
Research Org:
Thomas Jefferson National Accelerator Facility, Newport News, VA (United States)
Sponsoring Org:
USDOE Office of Science (SC), Nuclear Physics (NP) (SC-26)
Contributing Orgs:
The CLAS Collaboration
Country of Publication:
United States
Language:
English
Subject:
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS