First measurement of the polarization observable E in the (p)over-right-arrow((gamma)over-right-arrow, pi(+))n reaction up to 2.25 GeV

L. El Fassi
K. Hafidi
C. I. Moody
K. A. Griffioen, College of William and Mary


First results from the longitudinally polarized frozen-spin target (FROST) program are reported. The double-polarization observable E, for the reaction (p) over right arrow((gamma) over right arrow, pi( + ))n, has been measured using a circularly polarized tagged-photon beam, with energies from 0.35 to 2.37 GeV. The final-state pions were detected with the CEBAF Large Acceptance Spectrometer in Hall B at the Thomas Jefferson National Accelerator Facility. These polarization data agree fairly well with previous partial-wave analyses at low photon energies. Over much of the covered energy range, however, significant deviations are observed, particularly in the high-energy region where high-L multipoles contribute. The data have been included in new multipole analyses resulting in updated nucleon resonance parameters. We report updated fits from the Bonn-Gatchina, Jfilich-Bonn, and SAID groups. (C) 2015 The Authors. Published by Elsevier B.V.