$\eta$' meson production in the pp $\rightarrow$ pp$\eta$' reaction near threshold

The $\eta$' meson belongs to the ground starte 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 ($\Theta_{PS} \approx$ 15°) the $\eta$' meson is predominantly a flavour singlet starte and hence can comprise a significant amount of gluons. A possible gluon adrnixture would he reflected in the production mechanism of the $\eta$' meson in the elernentary 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 malte the investigations of the $\eta$' meson very interesting. The total cross section for the production of the $\eta$' meson in the pp $\rightarrow$ pp$\eta$' reaction has been measured dose to the kinematical threshold. The experiment has been carried out at the cooler synchrotron COSY using the internat proton beamand the hydrogen cluster target. The ejected charged particles were registered using the COSY - 11 detection svstem, which allow for an unique identification of positively charged particles ancl determina tion of their four - mornentum vectors. Events wich 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 srnaller that the cross section for the production of the $\eta$ meson at the samecenter of mass excess energies. Such large difference suggests that diese mesons are produced in a rather different way. Comparison of the measured cross section values with calculations based an the effective Lagrangian approach yields an upper limit for the $\eta$'pp coupling constant. The analysis of the energy dependence of the total cross section for the pp $\rightarrow$ pp$\eta$' reaction suggests that either the primary production amplitude decreases very dose to threshold or that the proton-$\eta$' interaction is repulsive