GRAAL: Photoreactions by Means of Polarized Gamma Rays
According to the Standard Model, the taxonomy of the elementary particles starts from the fundamental constituents, leptons and quarks. The first experiments in pion nucleon scattering led quickly to the identification of intermediate, short-living, nuclear states, the baryon resonances. Furthermore, the underlying structure of nucleons by means of internal degrees of freedom was confirmed by several experimental results on electromagnetic and weak scattering on the nucleon.
Nowadays, polarized high energy photons are excellent probes of protons, neutrons, and nuclei, despite their coupling is limited solely to electric charge and magnetic moment.
The extraction of photo-excitation amplitudes beyond the first resonance region (Ecmtot ≈ 1.5 GeV) constitutes a powerful tool in the search for the so-called ‘missing’ resonances, predicted by QCD inspired models, but not observed in conventional πN→ πN scattering experiments. Thanks to the recent development of new facilities which use a polarized beam and/or target combined with large-acceptance–high granularity detectors, the experimental database has significantly improved.
The GRAAL (for “GRenoble Anneau Accelerateur Laser”) facility (see Activity report pag 91) provides a tagged polarized γ-ray beam, while a very complex detection apparatus (fig. 1) covering almost the whole solid angle authorizes the measurement of energy loss and time of flight of charged and neutral particles. With its maximum beam energy of ~ 1.6 GeV, the GRAAL apparatus allows for the study of all baryon resonances with masses above the Δ and up to ~1.8 GeV.
V. Bellini, A. Giusa, F. Mammoliti, C. Randieri, G. Russo, M. L. Sperduto, C. M. Sutera