Near-threshold production of the {eta} meson via the quasifree pn{yields}pn{eta} reaction

Total cross sections for the quasifree pn{yields}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{yields}pp{eta} reaction by a factor of about 3 in contrast to the factor of 6 established for higher excess energies. To large extent, the observed decrease of the ratio {sigma}(pn{yields}pn{eta})/{sigma}(pp{yields}pp{eta}) toward threshold may be assigned to the different energy dependence of the proton-proton and proton-neutron final-state interactions. The experiment was 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{yields}pn{eta} reaction have been registered by means of the COSY-11 facility, an apparatus dedicated for threshold meson production.