STUDY OF THE THRESHOLD BEHAVIOR OF THE ηN SCATTERING AMPLITUDE THROUGH THE ASSOCIATED PHOTOPRODUCTION OF ϕ- AND η-MESONS

We suggest that the γp → ϕηp reaction cross section, in the kinematics where the ηp invariant mass in the final state lies between the threshold value (m_p+m_η) and the N*(1535) resonance mass, is largely determined by the ηN scattering amplitude close to threshold. The initial photon energy is chosen in the range , in order to reach low (absolute) values of the squared 4-momentum transfer from the initial photon to the final ϕ-meson. In these conditions, we expect the t-channel π⁰- and η-meson exchanges to drive the dynamics underlying the γp → ϕηp process. We show that the η-exchange is the dominating contribution to the cross section while the π⁰-exchange is negligible. The η-π⁰ interference is of the order of 20 – 30%. The sign of this term is not known and alters significantly our results. Data on the γp → ϕηp process would be therefore very useful to help unravelling the behavior of the ηp scattering amplitude close to threshold and assessing the possibility of producing η-nucleus bound states.