ω PHOTOPRODUCTION AT GRAAL

The study of meson photoproduction on the nucleon is a very important tool to complete the puzzle of baryon excited states. GRAAL aim is the measurement of cross sections and of beam asymmetries for all the photonuclear reactions accessible in the energy range from 600 MeV up to 1.5 GeV. In the following preliminary results of the Σ beam asymmetry for ω photoproduction on the free proton in Hydrogen and on the quasi-free nucleon in Deuterium are shown. GRAAL is the first experiment in which both the charged and the radiative decay of ω meson are studied. Since the beam asymmetry is independent of the decay mode, the comparison of the results from the two channel allows an important check on data consistency. Results on the free proton are in good agreement with theoretical prediction from Zhao model and confirm the presence of P13(1720) state. The analysis performed for the radiative decay is applied to the process of ω photoproduction on the quasi free nucleon in Deuterium.