Production of η-mesons in proton-neutron collisions

Production of η-mesons in p-n collisions has been studied using a deuterium internal cluster-jet target and the CELSIUS storage ring in Uppsala. The measurements were performed with the WASA/PROMICE setup, a detector setup designed to take advantage of the kinematical properties of meson production close to threshold. By exploiting the Fermi momentum of the neutron in the deuteron, the excitation functions for the reactions p+n→p+n+η and p+n->d+η have been measured for excess energies in the centre-of-mass system from 15 to 115 MeV staying at a fixed proton-beam energy of 1.3 to 1.55 GeV for a target neutron at rest. The measured energy dependence of the cross section for the p+n→D+η reaction is the first firm evidence of the importance of the N*(1535) nucleon resonance in the production mechanism. The cross section for the reaction p+n→p+n+η was found to be at least six times larger than that of the reaction p+p→p+p+η. This is surprisingly high since it is at least a factor of two larger than the existing theoretical predictions. 56 refs