At the GRAAL facility, a polarised and tagged (gamma) over right arrow ray beam is produced in the energy range from 500 MeV up to 1500 MeV. Preliminary results of beam polarization asymmetries for Compton scattering on the proton are presented. These very precise measurements cover the angular range 30degrees-150degrees, providing stringent constraints to theoretical models.

A polarized and tagged gamma ray beam is produced at GRAAL by the Compton scattering of laser light on the high energy electron circulating in the ESRF storage ring. We present results on the beam polarization asymmetries in the Compton Scattering and positive pion photo-production on proton target in the energy region 500-1500 MeV. These very precise results cover the angular range 30-150 degrees, providing stringent constraints to theoretical models.

A polarised and tagged \(\vec \gamma \) ray beam is produced at GRAAL with energies ranging from 500 MeV up to 1500 MeV. Results of beam polarisation asymmetries and cross sections for the photoproduction of pseudoscalar mesons on the proton are presented. These very precise measurements cover the angular range 30°–150°, providing stringent constraints to theoretical models.

Since 1996 the GRAAL experiment is running at the ESRF, using a Linearly polarized photon beam produced by backscattering a laser beam on the electron beam in the storage ring. A liquid hydrogen target and a large acceptance detector are used. All sizeable photoproduction reactions are measured and analysed. We present the results of eta meson, one and two pions, and omega photoproduction, which are partly published or still preliminary. Predictions or fits of these data by theoretical models are given.

The features of the Graal experiment at ESRF are presented, The obtained results are discussed and future developments outlined.

The GRAAL experiment at the ESRF is presented. The experimental setup is described with its main characteristics. Beam asymmetry Sigma measurements in the main channels together with their interest are given.

Sigma beam asymmetries for pi(+) photoproduction on the proton have been measured in the energy range 550-1100 MeV over a wide angular range. The statistical and systematic errors of the data are much lower than in previous measurements, due to the high polarization of the photon beam and the cylindrical geometry of the 4 pi detector. Our results confirm the predictions W198 and SM95 of the VPI partial-wave analysis at the energies below 850 MeV and at 1050 MeV while, between these two regions, they differ at backward angles. It may indicate the smaller contribution of S-11,,(1650) resonance, than it was quoted previously by the VPI and PDG groups. A newly developed unitary isobar model reproduces our data up to 950 MeV. (C) 2000 Elsevier Science B.V. All rights reserved.

We present new Sigma beam asymmetry data for eta meson photoproduction on the proton, using a novel tagged, laser backscattered, linearly polarized photon beam up to 1.1 GeV. The data show large, positive asymmetries, at all incident photon energies. In addition to the S-11(1535) and D-13(1520) resonances necessary to reproduce the cross sections, P-13(1720) and D-15(1675) "four stars" resonances contribute to the Sigma observable, but cannot reproduce the strong forward asymmetries measured at energies higher than 900 MeV.

We present new $\ensuremath{\Sigma}$ beam asymmetry data for $\ensuremath{\eta}$ meson photoproduction on the proton, using a novel tagged, laser backscattered, linearly polarized photon beam up to 1.1 GeV. The data show large, positive asymmetries, at all incident photon energies. In addition to the ${S}_{11}(1535)$ and ${D}_{13}(1520)$ resonances necessary to reproduce the cross sections, ${P}_{13}(1720)$ and ${D}_{15}(1675)$ ``four stars'' resonances contribute to the $\ensuremath{\Sigma}$ observable, but cannot reproduce the strong forward asymmetries measured at energies higher than 900 MeV.