Abstract
A summary is presented on measurements of J / ψ meson production in electron proton collisions, measured with the H1 and ZEUS detectors at HERA. J / ψ mesons are detected by their leptonic decay modes into electrons and muons. The photoproduction cross section for elastic J / ψ meson production is observed to rise with the centre-of-mass energy, exceeding the expectations of diffractive models based on soft pomeron exchange. The results on inelastic J / ψ photoproduction are compared with NLO QCD calculations based on the colour singlet model. The energy dependence of the cross section is found to agree in shape with the NLO QCD predictions, provided the gluon distribution in the proton is assumed to rise strongly towards low x . In deep inelastic scattering the quasi-elastic J / ψ production γ * p cross section is measured at an average Q 2 ≈ 18 GeV 2 and W γ p = 70 GeV. It is found to be of order 20 nb, large compared with data at lower energies.
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