Photoproduction of η mesons on protons in the resonance region: The background problem and the third S 11 resonance. V.A. TryasuchevFebruary 28, 2018 AbstractWe have constructed an isobar model for the η-photoproduction on the proton in the energy region up to the photon lab energy K0 = 3 GeV. The data base involved into the fitting procedure includes precise results for the cross section and for the T -asymmetry of the process γp → ηp near threshold obtained at MAMI and ELSA as well as recent results for the Σ-asymmetry and for the angular distribution measured at higher energies in Grenoble and also more recent measurements performed at JLab for the photon energies up to 2 GeV. The model includes twelve nucleon resonances: S11(1535), S11(1650), S11(1825), P11(1440), P13(1720), D13(1520), D15(1675), F15(1680), F17(1990), G17(2190), G19(2250), H19(2220), and the background consisting of the nucleon pole term and the vector meson exchange in the t-channel. To explain the observed energy dependence of the integrated cross section, two s-wave resonances, S11(1650) and S11(1825), have to be taken into account along with the dominating S11(1535). The integrated cross section as well as the angular distribution and Σ asymmetry predicted by the model are in good agreement with the data. Above the photon energy K0 = 2 GeV, the calculated cross section exhibits an appreciable dependence on the ρ-and ω-meson contribution, whose coupling with nucleons is not well defined. Several versions of extending the model to higher energies are considered.

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The cross sections for production of eta nuclei (eta A) by photons are calculated in the framework of the potential nonrelativistic model. The total cross sections of the gamma +12C to p+eta 11B and delta +16O to p+eta 15N reactions characterized by predominant forward emission of protons are shown to take on the maximum values of 0.4 mu b and 1.5 mu b, respectively, at a 0.75 GeV photon energy. The photoproduction of eta nuclei must give rise to characteristic peaks in the nucleon spectra of the gamma A to pX reactions.

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