Three - pair final - state interaction in the pp → pp η reaction close to threshold 1 )

We present a three-body formalism describing the final-state interaction effects in the pp → ppη reaction close to threshold. We derive a three-body enhancement factor devised in such a way that all three pair-wise interactions are regarded on equal footing. The enhancement factor is obtained by expanding the three-particle wave function in hyperspherical harmonics. It has been shown that close to threshold the p − p interaction strongly dominates whereas the η − p interaction gives almost negligible contribution to the calculated effective mass spectra. Within the presented three-body approach it has been possible to reproduce the effective mass distributions at the excitation energy Q=15.5 MeV in good accord with the data.