Hadronic form factors and the $J/\psi$ secondary production cross   section: an update
  Improving previous calculations, we compute the $D + \bar{D} \to J/\psi + \pi $ cross section using the most complete effective lagrangians available. The new crucial ingredients are the form factors on the charm meson vertices, which are determined from QCD sum rules calculations. Some of them became available only very recently and the last one, needed for our present purpose, is calculated in this work. 
F. Carvalho, F.O. Dur\~aes, F.S. Navarra and M. Nielsen