Interplay of Interference Effects in Production of $J/\Psi$ and $\Psi'$   off Nuclei
  Our main observations are: (i) The effective path length of the $c\bar c$ wave packet, which is produced in the nucleus, grows with the energy of the produced charmonium. The variation is controlled by the coherence length $l_c=2E_{\Psi}/M^2_{\Psi}$. (ii) A colorless $c\bar c$ wave packet produced in $pp$-interaction is a specific linear combination of the $J/\Psi$ and $\Psi'$ states, and interacts substantially weaker that any of its components. The time evolution of this wave packet is controlled by the formation length, $l_f=2E_{\Psi}/(M^2_{\Psi'} - M^2_{\Psi})$. Exact formulas incorporating with these effects are derived. The interplay of the two phenomena results in a nontrivial energy- and $x_F$-dependence of the nuclear suppression of the charmonium production in proton-nucleus and nucleus-nucleus collisions and explains some of the experimentally observed effects. 
Joerg Huefner (U. Heidelberg), Boris Kopeliovich (MPI-K Heidelberg)