Measurement of the cross section for the reaction γp → J/ψ p with the ZEUS detector at HERA

This paper reports the cross section measurements for the process ep → e J/ψ p for Q2 < 4 GeV2 at s = 296 GeV, based on an integrated luminosity of about 0.5 pb−1, using the ZEUS detector. The J/ψ was detected in its e+e− and μ+μ− decay modes. The photoproduction cross section was measured to be 52−12+7 ± 10 nb at an average γp centre of mass energy of 67 GeV and 71−20+13±12 nb at 114 GeV. The significant rise of the cross section compared to lower energy measurements is not in agreement with VDM models, but can be described by QCD inspired models if a rise in the gluon momentum density at low x in the proton is assumed.