$η′$ production in proton‐proton collisions far from the threshold

I discuss exclusive production of the $η′$ meson in the $pp→pη′p$ reaction far from the threshold. The contribution of diffractive component as well as that for $γ*γ*→η′$ fusion are calculated. In the first case the formalism of unintegrated gluon distribution functions (UGDF) is used. The distributions in the Feynman $xF$ (or rapidity), transferred four‐momenta squared between initial and final protons $(t1,t2)$ and azimuthal angle difference between outgoing protons (Φ) are calculated. The deviations from the $sin2(Φ)$ dependence predicted by one‐step vector‐vector‐pseudoscalar coupling are quantified and discussed. The results are compared with the results of the WA102 collaboration at CERN. Most of the models of UGDF from the literature give too small cross section as compared to the WA102 data and predict angular distribution in relative azimuthal angle strongly asymmetric with respect to $π/2$ in disagreement with the WA102 data. This points to a different mechanism at the WA102 energy. Predictions for RHIC, Tevatron and LHC are given.