First Measurement of the Polarization Observable E in the $\vec p(\vec \gamma,\pi^+)n$ Reaction up to 2.25 GeV

TL;DR

This paper reports the first measurement of the polarization observable E in the reaction involving polarized protons and photons, providing new data that refine nucleon resonance models up to 2.25 GeV.

Contribution

It presents the first measurement of the polarization observable E in the specified reaction, expanding the experimental data available for nucleon resonance analysis.

Findings

Data agree with previous analyses at low energies

Significant deviations observed at higher energies

Updated resonance parameters from new multipole analyses

Abstract

First results from the longitudinally polarized frozen-spin target (FROST) program are reported. The double-polarization observable E, for the reaction $\vec \gamma \vec p \to \pi^+n$, 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, J\"ulich, and SAID groups.