We would like to continue the comparative study of the interactions of the eta and eta' mesons with protons as suggested in the already accepted COSY Proposal No. 11.7 titled "Study of proton-eta' and proton-eta interaction". The first part of these investigations, namely the study of the proton-eta interaction, is finished. Here we present results of the completed evaluation of the high statistics measurement of the pp->pp eta reaction performed at the COSY-11 facility during the beam-time which had been assigned in the frame of that proposal. Qualitative phenomenological analysis of the determined differential distributions for the production cross sections revealed an enhancement of the population density in the kinematical regions corresponding to the low proton-eta relative momentum. The effect is too large to be described by the on-shell incoherent pairwise interaction among the produced particles and its understanding will require a rigorous three-body approach to the pp eta system. At present many theoretical groups are seeking an explanation of this intriguing observation.
Encouraged by this finding we intend now to realize the second part of the Proposal No. 11.7, that is to investigate the interaction of the meson eta' with protons. Additionally to the originally presented motivation for the study of the low energy pp eta' system we argue that the comparison of the differential cross sections of the pp->pp eta' and pp->pp eta reactions combined with the shape of the excitation functions will facilitate to disentangle between the contributions from the higher partial waves and the proton-proton-meson interactions for both produced systems.
In the close to threshold excitation function for the pp->pp eta' reaction a signal from the proton-eta' force has not been observed. Thus, the determination of the distributions of the invariant masses for two-particle subsystems of the pp eta' final state could give the first ever experimental evidence for this still completely unknown interaction. In the case of the pp eta system these observables proved to be very sensitive to the proton-eta interaction. Their determination would also strengthen significantly constraints on the analysis aiming at the determination of contributions from mesonic, gluonic, nucleonic and resonance currents to the production process. Therefore, we ask for 4 weeks of COSY beam-time in the coming period in order to determine differential distributions of the cross section for the pp->pp eta' reaction at Q = 15.5 MeV, the same excess energy at which the corresponding spectra for the proton-eta reaction have been determined.
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