The eta' meson belongs to the ground state pseudoscalar meson nonet. According to the SU(3) classification its quark composition is similar to the structure of the eta meson. However, due to the small pseudoscalar mixing angle (ThetaPS~15°) the eta' meson is predominantly a flavour singlet state and hence can comprise a significant amount of gluons. A possible gluon admixture would be reflected in the production mechanism of the eta' meson in the elementary proton-proton reaction, it would modify an eta' pp coupling constant which determines the direct production amplitude, and it would also influence the potential of the proton-eta' meson interaction. These issues make the investigations of the eta' meson very interesting.
The total cross section for the production of the eta' meson in the pp->pp eta' reaction has been measured close to the kinematical threshold. The experiment has been carried out at the cooler synchrotron COSY using the internal proton beam and the hydrogen cluster target. The ejected charged particles were registered using the COSY-11 detection system, which allow for an unique identification of positively charged particles and determination of their four-momentum vectors. Events with the creation of the eta' meson were identified by means of the missing mass technique.
The obtained cross sections for the eta' meson production are by about a factor of fifty smaller that the cross section for the production of the eta meson at the same center of mass excess energies. Such large difference suggests that these mesons are produced in a rather different way.
Comparison of the measured cross section values with calculations based on the effective Lagrangian approach yields an upper limit for the eta' coupling constant.
The analysis of the energy dependence of the total cross section for the pp->pp eta' reaction suggests that either the primary production amplitude decreases very close to threshold or that the proton-eta' interaction is repulsive.
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