It is well established that the eta meson in the nucleon-nucleon collisions is produced via the excitation and subsequent decay of the N*(1535) resonance. The WASA/PROMICE collaboration have found that value of the total cross section for the reaction pp -> pp eta is about 6.5 times smaller than that for the pn -> pn eta reaction in the excess energy range between 16 MeV and 109 MeV [1]. This observation is explained by an assumption that the eta meson is produced via dominant isovector meson (pi,rho) exchange between the interacting nucleons.
To check the behavior of the cross section ratio closer to threshold (from 0 to 20 MeV) we have studied both above mentioned reactions using the COSY-11 facility.
The COSY-11 detection setup allows for the detection of four-momentum vectors for all baryons produced in the reactions pp -> pp eta and pd -> psppn eta. In the latter process it is possible to determine the excess energy of the quasi-free pn -> pn eta reaction for each registered event [2]. However, an identification of the pn -> pn eta reaction on the event-by-event basis is impossible if only the four-momentum vectors of nucleons are measured. Therefore, the analysis of the data required an elaboration of a new method allowing to distinguish the production of eta from the multi-pion background.
This techniqe will be presented and discussed. We will also present preliminary experimental results from the eta meson production in quasi-free proton-neutron scattering process.
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