Inelastic J ψ-Photoproduction at HERA and the Color Evaporation Model

An investigation of the color evaporation model is presented. The color evaporation model describes the production of J ψ mesons. The numerical FMNR program is used to calculate the next to leading order J ψ production cross sections as predicted by the color evaporation model. Cross sections are calculated as functions of pT , Wγp and z. In addition the p 2 T distribution was investigated in four different z regions. The color evaporation model predictions with and without a phenomenological correction factor introduced by Halzen et al. are investigated. Both are compared with a color singlet model Monte-Carlo generator and with data collected at the HERA collider. It is found that the data for the pT , Wγp and z distributions are in a good agreement with the color evaporation model predictions with the correction factor. The shapes of the double differential pT distributions for the data however are not described by the corrected distributions of the color evaporation model. For the double differential distributions the predictions without the correction factor are in very good agreement with the data.