Measurement of ep→e′nπ⁺ at 1.6<W<2.0 GeV and extraction of nucleon resonance electrocoupling at CLAS

Spokespersons

K. Park, V.D. Burkert, I.G. Aznauryan

Year

2014

Abstract

The differential cross sections of exclusive process $ep \toe\pi^+n$ were measured in the range of the photon virtuality Q2 = 1.8-4.5GeV2, and the invariant mass range of the $\pi+n$ final state W = 1.6-2.0 GeV using the CEBAF Large Acceptance Spectrometer. Full angular distributions in azimuthal and polar angles were mapped out with good precision. More than 37,000 cross section points were measured. The contributions of the isospin I = 1/2 resonances N(1675)5/2-,N(1680)5/2+, and N(1710)1/2+ 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. Differences resulting from these independent approaches are taken as model sensitivity of the analysis. We observe significant strength of the N(1675)5/2- in the A1/2 amplitude even at high Q2, in strong disagreement with quark models that predict both transverse amplitudes to be consistent with zero. For the N(1680)5/2+ we observe an unexpectedly 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, indicating significant strength from non-quark contribution even at large Q2. The scalar amplitude S1/2 drops rapidly with Q2 consistent with quark model prediction. For the N(1710)1/2+ resonance our analysis shows significant strength for the A1/2 amplitude at Q2 < 2.5GeV2.

Comment

polarization of electron beam : ~80%

Publication

1. CLAS-Analysis Note 2014-105 arXiv:1412.0274

Quantities

dσ/dΩ

Measurements