Near-threshold production of theηmeson via the quasifreepn→pnηreaction

Total cross sections for the quasifree pn→pnη 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→ppη 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 σ(pn→pnη)/σ(pp→ppη) 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→pnη reaction have been registered by means of the COSY-11 facility, an apparatus dedicated for threshold meson production.Received 15 October 2008DOI:https://doi.org/10.1103/PhysRevC.79.015208©2009 American Physical Society