Measurement of the \Sigma\ beam asymmetry for the \omega\ photo-production off the proton and the neutron at GRAAL

TL;DR

This paper presents new measurements of the \

Contribution

It provides the first simultaneous analysis of radiative and three-pion decay channels for \

Findings

Agreement between decay channels on proton measurements

Differences in beam asymmetry between proton and neutron targets

First data on quasi-free neutron asymmetry

Abstract

We report on new measurements of the beam asymmetry for \omega\ photo-production on proton and neutron in Hydrogen and Deuterium targets from the GRAAL collaboration. The beam asymmetry values are extracted from the reaction threshold (E_{\gamma} = 1.1 GeV in the free nucleon kinematics) up to 1.5 GeV of incoming photon energy. For the first time both the radiative and the three-pion decay channels are simultaneously investigated on the free proton. Results from the two decay channels are in agreement and provide important constraints for the determination of resonant state contributions to the \omega\ production mechanism. First experimental results on the deuteron allow the extraction of the \Sigma\ beam asymmetry on quasi-free nucleons. Comparison of the results for free and quasi-free kinematics on the proton shows a generally reasonable agreement, similar to the findings in pseudo-scalar meson photo-production reactions. For the first time measurements on quasi-free neutrons are available, showing that both the strength and the angular distributions of the beam asymmetry are sensibly different from the results on the proton target.