Total cross sections for the quasi-free pn --> pn eta reaction in the range from the kinematical threshold up to 20 MeV excess energy have been determined. At threshold they exceed corresponding cross sections for the pp --> pp eta reaction by a fact
or of about three in contrast to the factor of six established for higher excess energies. To large extent, the observed decrease of the ratio sigma(pn --> pn eta)/sigma(pp --> pp eta) towards threshold may be assigned to the different energy dependence of the proton-proton and proton-neutron final state interactions. The experiment has been conducted using a proton beam of the cooler synchrotron COSY and a cluster jet deuteron target. The proton-neutron reactions were tagged by the spectator proton whose momentum was measured for each event. Protons and neutron outgoing from the pn --> pn eta reaction have been registered by means of the COSY-11 facility, an apparatus dedicated for threshold meson production.
We summarise measurements of the COSY-11 collaboration concerning the excitation functions of the near threshold pseudoscalar meson production in the proton-proton interaction. The results are discussed in the context of the meson-nucleon and hyperon
-nucleon interactions. We conclude that the $eta$-proton interaction is significantly stronger than the eta-proton interaction. Similarly, we found that the hyperon Lambda interacts with the nucleon considerably stronger than the hyperon Sigma, and that the interaction of (K-)-proton is much stronger than this of the (K+)-proton.
We present the results of measurements of the analysing power for the p(pol)p --> pp eta reaction at the excess energies of Q=10 and 36 MeV, and interpret these results within the framework of the meson exchange models. The determined values of the a
nalysing power at both excess energies are consistent with zero implying that the eta meson is produced predominantly in s-wave.
