Photoproduction of {\omega} Mesons off the Proton

TL;DR

This paper reports comprehensive measurements of differential cross sections and spin-density matrix elements for omega photoproduction off protons, revealing the roles of various resonances and exchange mechanisms through new polarized data and partial wave analysis.

Contribution

It provides the first use of linearly polarized photons in this energy range and integrates these data into a detailed partial wave analysis to identify contributing resonances and exchange processes.

Findings

Dominant omega production near threshold is from the 3/2^+ partial wave due to N(1720) resonance.

At higher energies, pomeron-exchange dominates with small pi exchange.

Multiple nucleon resonances contribute significantly to the reaction mechanism.

Abstract

The differential cross sections and unpolarized spin-density matrix elements for the reaction $\gamma p\to p\omega$ were measured using the CBELSA/TAPS experiment for initial photon energies ranging from the reaction threshold to 2.5 GeV. These observables were measured from the radiative decay of the $\omega$ meson, $\omega\to\pi^0\gamma$. The cross sections cover the full angular range and show the full extent of the $t$-channel forward rise. The overall shape of the angular distributions in the differential cross sections and unpolarized spin-density matrix elements are in fair agreement with previous data. In addition, for the first time, a beam of linearly-polarized tagged photons in the energy range from 1150 MeV to 1650 MeV was used to extract polarized spin-density matrix elements. These data were included in the Bonn-Gatchina partial wave analysis (PWA). The dominant contribution to $\omega$ photoproduction near threshold was found to be the $3/2^+$ partial wave, which is primarily due to the sub-threshold $N(1720)\,3/2^+$ resonance. At higher energies, pomeron-exchange was found to dominate whereas $\pi$-exchange remained small. These $t$-channel contributions as well as further contributions from nucleon resonances were necessary to describe the entire dataset: the $1/2^-$, $3/2^-$, and $5/2^+$ partial waves were also found to contribute significantly.