The GRAAL facility produces a highly polarized gamma-ray beam by Compton scattering of laser photons on the electrons of the European Synchrotron Radiation Facility (ESRF) at Grenoble. Preliminary results have been obtained with the LAGRANGE detector showing its excellent performances

The Sigma beam asymmetry in the photoproduction of neutral pions from quasi-free nucleons in a deuteron target was measured for the first time between 0.60 and 1.50 GeV, with the GRAAL polarized and tagged photon beam. The asymmetry values from the quasi-free proton were found equal to the ones extracted from a pure proton target. The asymmetries from quasi-free proton and quasi-free neutron were found equal up to 0.82 GeV and substantially different at higher energies. The results are compared with recent partial-wave analyses.

The gamma p -> eta'p reaction was studied near the threshold, in the 1.44 GeV > E(gamma) < 1.55GeV energy range with the tagged photons of the former GRAAL experiment. The two neutral decays gamma gamma(Gamma(i)/Gamma = 2.10 +/- 0.12%) and pi(0)pi(0)eta(Gamma(i)/Gamma = 20.7 +/- 1.2%) were analized and preliminary results of the beam asymmetry were extracted together with the invariant masses.

Beam asymmetry and differential cross-section for the reaction gamma p -> eta p were measured from production threshold to 1500MeV photon laboratory energy. The two dominant neutral decay modes of the eta-meson, eta -> 2 gamma and eta -> 3 pi(0) , were analyzed. The full set of measurements is in good agreement with previously published results. Our data were compared with three models. They all fit satisfactorily the results but their respective resonance contributions are quite different. The possible photoexcitation of a narrow state N(1670) was investigated and no evidence was found.

Compton backscattering gamma ray beams are characterized by a high degree of linear and circular polarization with low unpolarized backgrounds and have proven to provide very precise measurements of polarization observables. Latest results from LEGS and GRAAL experiments on proton and deuteron targets are presented. The Sigma beam asymmetry for omega photoproduction has been measured by the GRAAL collaboration for both the omega -> pi(0)gamma and the omega -> pi(+)pi(+)pi(0) decay channels on the proton target; single and double polarization asymmetries have been provided also for the K photoproduction channel. E and G double polarization asymmetries for single pion photoproduction on the proton and deuteron have been measured at LEGS using a frozen spin HD target.

The preliminary results obtained by the GRAAL collaboration for the pi(0)pi(+) photoproduction on the free and quasi-free proton (deuteron) at E(gamma) = 0.7-1.5 GeV are presented. The total cross section of the gamma p -> pi(0)pi(+)n reaction and invariant mass spectra for the pi(+)pi(0), pi(+)n and pi(0)n systems are presented in the photon energy range from 0.7 to 1.5 GeV. These results are in good agreement with the 2 pi-MAID calculations.

Beam asymmetries and hyperon recoil polarizations for the reactions gamma p --> K+ Lambda and gamma p --> K+ Sigma(0) have been measured from the threshold production to 1500 MeV with the GRAAL facility located at the ESRF in Grenoble. These results complement the database for the beam asymmetry, covering for the first time the production threshold region. Recent theoretical analyses are presented for which the beam asymmetry data bring interesting new information and allow to better determine some resonance parameters. Most importantly, these results strengthen the need of a new D-13 state around 1900 MeV.

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 Sigma beam asymmetry for omega 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 omega 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 P(13)(1720) state. The analysis performed for the radiative decay is applied to the process of. photoproduction on the quasi free nucleon in Deuterium.

The Sigma beam asymmetry in eta' photoproduction off the proton was measured at the GrAAL polarised photon beam with incoming photon energies of 1.461 and 1.480 GeV. For both energies the asymmetry as a function of the meson production angle shows a clear structure, more pronounced at the lowest one, with a change of sign around 90 degrees. The observed behaviour is compatible with P-wave D-wave (or S-wave F-wave) interference, the closer to threshold the stronger. The results are compared to the existing state-of-the-art calculations that fail to account for the data.

We present the analysis of data performed in order to identify the events of the gamma + n -> pi(-) + p reaction obtained by bombarding a liquid Deuterium target with a polarised. beam of 0.55-1.5 GeV at the Graal-experiment. We show the effect of different kinematic and hardware constraints used to reduce the contamination coming from the concurrent reaction channels. By the simulation we estimate the contamination degree due to the other reaction channels so we can test the reliability of our method. We describe a new three-dimensional cut based on the Fermi momentum reconstruction and its effect on the suppression of the concurrent double charged pion photoproduction. We present the preliminary beam asymmetry Sigma of the pi(-) fotoproduction off quasi-free neutron up to about theta(c.m.), (pi-) = 165 degrees together with some theoretical multipolar analysis. For a comparison we also report the data present in literature on the same reaction for E(gamma) -850-1740 MeV and theta(c.m.), (pi-) <= 105 degrees.

The extraction of resonance parameters from meson photo-reaction data is a challenging effort, that would greatly benefit from the availability of several polarization observables, measured for each reaction channel on both proton and neutron targets. In the aim of obtaining such complete experiments, polarized photon beams and targets have been developed at facilities, worldwide. We report on the latest results from the LEGS and GRAAL collaborations, providing single and double polarization measurements on pseudo-scalar meson photo-production from the nucleon.

The double polarization (beam-recoil) observables O(x) and O(z) have been measured for the reaction gamma p -> K(+) Lambda from threshold production to E gamma similar to 1500 MeV. The data were obtained with the linearly polarized beam of the GRAAL facility. Values for the target asymmetry T could also be extracted despite the use of an unpolarized target. Analyses of our results by two isobar models tend to confirm the necessity to include new or poorly known resonances in the 1900 MeV mass region.

New experimental approach to study meson photoproduction in nuclei is discussed. It is based on the measurement of the recoil nucleon as a tagger for identification of the initial step of reaction. Propagation of mesons and nucleons within a nucleus is described by the Intra-Nuclear Cascade (INC) model. Simulations and preliminary experimental data for the deuteron target obtained at GRAAL are presented. (c) 2008 Elsevier B.V. All rights reserved.

The photoproduction of double pi(0) on the neutron is studied in the beam energy range of 0.6 up to 1.5 GeV, using a liquid deuterium target. The cross section and the beam asymmetry are extracted and compared to those previously obtained on a proton target. The theoretical interpretation of these results is given using different models. (c) 2007 Elsevier B.V. All rights reserved.