- th / 9 80 60 22 v 2 2 2 Ju n 19 98 η ′ Production in Proton - Proton Collisions near Threshold ∗

Within the one-pion exchange model we calculate the near-threshold η ′ production from pp collisions including the final state interaction between the protons. Since the description of the data is quite well we conclude that η, ρ or ω exchange currents either play no role or cancel each other to a large extent in this reaction. Recently the COSY-11 [1] and SPES III [2] Collaborations have measured the η ′ production in proton-proton collisions at excess energies ǫ = √ s − 2m N − m η ′ ≤ 10 MeV. As found in Ref. [2] the calculation for the pp → ppη ′ cross section within the one-pion-exchange model – based on a comparison between η and η ′ production amplitudes – underestimates the experimental data by up to a factor of 2, such that the ρ and other heavy-meson exchange diagrams [3] should contribute substantially for η ′ production. In this short note we will argue that this discrepancy vanishes when including the average amplitude |M πN →η ′ N | from experimental data while also taking into account the interaction between the protons in the final-channel. Here we calculate the η ′ production within the one-boson exchange model first neglecting the Final-State-Interaction (FSI). For the one-pion exchange the pp → ppη ′ production amplitude [4] then reads M = g N N π F (t) ¯ u(p 1)γ 5 u(p a) 1 t − µ 2 M π 0 p→pη ′ , (1) where g N N π = 13.59 [5] is the ppπ 0 coupling constant, t = (p a − p 1) 2 is the squared 4-momentum transfer from the initial to the final proton, µ is the pion mass and F (t) is the form factor for the NNπ vertex F (t) = Λ 2 − µ 2 Λ 2 − t (2) with a cutoff parameter Λ = 1.3 GeV in line with Ref. [6]. In principle, η, ρ and ω exchanges should also contribute as suggested by the analysis in Ref. [2], however, there