Abstract
Differential and total cross section measurements on eta' photoproduction were published by the CLAS Collaboration (M. Dugger et al., Phys. Rev. Lett. 96, 062001 (2006) and M. Williams et al., Phys.Rev.C80, 045213 (2009)) for center-of-mass energies from near the threshold up to 2.84 GeV, and by the CB-ELSA-TAPS Collaboration (V. Crede et al., Phys. Rev. C80, 055202 (2009)) up to 2.36 GeV and also making a precise threshold scan of the differential cross section in the 1446 -1527.4 MeV gamma beam energy range. However, the wide information about reaction cross sections are not sufficient to understand the role of resonances involved in the process. Different theoretical works stressed the importance to have also polarization observables in order to solve the ambiguity in the choice of the parameters used in their models. We present the analysis of the eta' photoproduction off the proton, identifying the meson via the gamma gamma, pi(0)pi(0)eta, and pi(+)pi(-)eta decay modes by using the GRAAL apparatus; and we show the preliminary GRAAL results on the beam asymmetry Sigma from the threshold (1.446 GeV) up to 1.5 GeV.
Highlights
Polarization observables (Σ, T, P) are sensitive via interference between the complex helicity amplitudes, and allow to reveal the small resonance contributions which remain hidden under some dominant contribution in the differential cross section [1,2,3,4]
Η photoproduction works like an isospin-filter for nucleon resonances spectrum simplifying the data interpretation and the search of excited states produced in the reaction
The η meson photoproduction is interesting because it offers the possibility to investigate the N∗ nucleon resonances, in the less explored higher N∗ mass region[6]
Summary
The detailed description of the photon-nucleon interaction requires a complete data set containing, at least, eight independent observables: the cross section, the three single polarization. The GRAAL collaboration thanks to the stable and high polarization degree of its γ beam and a 4π detector, published precise single polarization observables for different meson photoproduction channels off free and quasifree proton [12,13,14,15,16,17].
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