Measurement of the invariant mass distributions for the reaction at excess energy of

The proton–proton and proton–η′η′ invariant mass distributions have been determined for the pp→ppη′pp→ppη′ reaction at an excess energy of Q=16.4 MeVQ=16.4 MeV. The measurement was carried out using the COSY-11 detector setup and the proton beam of the cooler synchrotron COSY. The shapes of the determined invariant mass distributions are similar to those of the pp→ppηpp→ppη reaction and reveal an enhancement for large relative proton–proton momenta. This result, together with the fact that the proton–η interaction is much stronger that the proton–η′η′ interaction, excludes the hypothesis that the observed enhancement is caused by the interaction between the proton and the meson.