Measurement of the helicity asymmetry $E$ in $\omega\to\pi^+\pi^-\pi^0$ photoproduction

TL;DR

This study measures the helicity asymmetry E in omega photoproduction using polarized beams and targets, revealing dominant resonance contributions and the evolution of exchange mechanisms across energies.

Contribution

First measurement of the helicity asymmetry E in omega photoproduction with polarized beams and targets, providing new insights into resonance contributions and exchange processes.

Findings

Dominant contribution from $3/2^+$ partial wave near threshold.

Pomeron exchange becomes dominant above 2 GeV.

Non-resonant $ ext{π}$ exchange remains small across energies.

Abstract

The double-polarization observable $E$ was studied for the reaction $\gamma p\to p\omega$ using the CEBAF Large Acceptance Spectrometer (CLAS) in Hall B at the Thomas Jefferson National Accelerator Facility and the longitudinally-polarized frozen-spin target (FROST). The observable was measured from the charged decay mode of the meson, $\omega\to\pi^+\pi^-\pi^0$, using a circularly-polarized tagged-photon beam with energies ranging from the $\omega$ threshold at 1.1 to 2.3 GeV. A partial-wave analysis within the Bonn-Gatchina framework found dominant contributions from the $3/2^+$ partial wave near threshold, which is identified with the sub-threshold $N(1720)\,3/2^+$ nucleon resonance. To describe the entire data set, which consisted of $\omega$ differential cross sections and a large variety of polarization observables, further contributions from other nucleon resonances were found to be necessary. With respect to non-resonant mechanisms, $\pi$ exchange in the $t$-channel was found to remain small across the analyzed energy range, while pomeron $t$-channel exchange gradually grew from the reaction threshold to dominate all other contributions above $W \approx 2$ GeV.