Near threshold production of the eta meson via the quasi-free pn --> pn   eta reaction
  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 factor 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. 
P. Moskal, R. Czyzykiewicz, H.-H. Adam, S. D. Bass, A. Budzanowski, E.   Czerwinski, D. Gil, D. Grzonka, M. Janusz, L. Jarcz...