Measurement of the cross-section for the reaction $\gamma p \to J/\psi p$ with the ZEUS detector at HERA

16 pp. (1995).
This paper reports the cross section measurements for the process ep → e J/ψ p for Q 2 < 4 GeV 2 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.