Quarkonium production in an improved color evaporation model

We propose an improved version of the color evaporation model to describe heavy quarkonium production. In contrast to the traditional color evaporation model, we impose the constraint that the invariant mass of the intermediate heavy quark-antiquark pair be larger than the mass of produced quarkonium. We also introduce a momentum shift between the heavy quark-antiquark pair and the quarkonium. Numerical calculations show that our model can describe the charmonium yields as well as the ratio of psi' over J/psi better than the traditional color evaporation model.