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 to be larger than the mass of produced quarkonium. We also introduce a momentum shift between heavy quark-antiquark pair and the quarkonium. Numerical calculations show that our model can describe the charmonium yields as well as ratio of $\psi^\prime$ over $J/\psi$ better than the traditional color evaporation model. 
Yan-Qing Ma and Ramona Vogt
hep-ph hep-ex