Photon- and pion asymmetry in near-threshold ω photoproduction

The photoproduction of ω mesons off protons has been studied from threshold to Eγ=1700 MeV in the reaction vector γp→ωp. The photon-beam asymmetry, Σ, and, for the first time, the pion asymmetry Σπ related to the ω→π0γ decay have been measured. The experiment was performed at the tagged photon beam of the ELSA electron accelerator of the University of Bonn. Using electron beams of E0=3.2 GeV linearly polarized photons were produced by coherent Bremsstrahlung. The ω meson was identified through it's neutral decay mode into π0γ. The corresponding three decay photons and the proton were detected in a combined setup of the Crystal Barrel and the TAPS calorimeters. Large photon asymmetries Σ in excess of 50 % were obtained. The pion asymmetries Σπ were found close to zero. This provides a new indication that, on top of t-channel exchange processes, baryonic s-channel resonances play an important role in ω photoproduction. The photoproduction of ω mesons thus continues to be a promising channel to find so far undetected baryonic resonances predicted by current quark models. Ongoing measurements at ELSA of further double polarization observables will be indispensable to fully disentangle the process.