Double Scattering Effect in Transverse Momentum Distribution of   Inclusive $J/\psi$ Production in Photo-Nucleus Collision
  In terms of multiple scattering picture, we calculate the double scattering effect in the transverse momentum distribution of $J/\psi$ photoproduction. Applying the generalized factorization theorem, we find that the contributions from double scattering can be expressed in terms of twist-4 nuclear parton correlation functions, which is the same as that used to explain the nuclear dependence in di-jet momentum imbalance and in direct photon production. Using the known information on the twist-4 parton correlation functions, we estimate that the double scattering contributes a small suppression in $J/\psi$ photoproduction. In the analysis we only take into account the leading order in the small velocity expansion for the nonperturbative parts related to the quarkonium. 
Han-Wen Huang