Photoproduction of ηmesons on protons in the resonance region:The background problem and the third S_11 resonance

Abstract

We have constructed an isobar model for the $η$-photoproduction on the proton in the energy region up to the photon lab energy $K_0 = 3$ GeV. The data base involved into the fitting procedure includes precise results for the cross section and for the $T$-asymmetry of the process $γp\toηp$ 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: $S_{11}(1535)$, $S_{11}(1650) $, $S_{11}(1825)$, $P_{11}(1440)$,$P_{13}(1720)$, $D_{13}(1520)$, $D_ {15}(1675)$, $F_{15}(1680)$, $F_{17}(1990)$,$G_{17}(2190)$, $G_{19} (2250)$, $H_{19}(2220)$, 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, $S_{11}(1650)$ and $S_{11}(1825)$, have to be taken into account along with the dominating $S_{11}(1535)$. 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 $K_0 = 2$ 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.