Abstract : The photoproduction of eta meson on hydrogen has been measured from threshold to 1100 MeV in GRAAL experiment performed in 1996-97 at the ESRF in Grenoble by a Collaboration of Italian, French and Russian groups.
A tagged and linearly polarized photon beam has been produced by backscattering a laser beam on the 6 GeV electron beam of the ring of the ESRF. The target was a liquid hydrogen target of 3 cm. A 4 pi detection system was used. It is composed of three layers of detectors: wire chambers, scintillator counters and calorimeters.
We had in charge to install and calibrate a double wall of scintillator bars 3x3 m designed to detect at forward angles (1-25°) the charged particles and to measure their loss of energy and their time of flight.
We analysed the results of the reaction g p -> eta p by identifying the eta in the lateral calorimeter which is a BGO ball and by detecting the proton in the scintillation counters.
The beam asymmetry Sigma was extracted from phi distribution of eta meson and was plotted against theta CM of eta for 6 intervals of energy between the threshold and 1100 MeV.
The interpretation of beam asymmetry Sigma was performed in the frame of the isobaric model. The use of the two resonances S11(1535) and D13(1520) to explain our results was not sufficient.
We had to take into account in addition the excitation of the resonances P13(1720) et D13(1675) et P13(1880) the last being missing in the table of resonances and recently revealed by quark models.