J/psi and eta_c in the nuclear medium: QCD sum rule approach
  We investigate the masses of the lowest c\bar{c} states, the J/psi and eta_c, in nuclear matter using QCD sum rules. Up to dimension four, the differences between the operator product expansions in vacuum and in medium arise from the density-dependent change in the gluon condensate and from a new contribution proportional to the nucleon expectation value of the twist-2 gluon operator. Both terms together give an attractive shift of about 5-10 MeV to the J/psi and eta_c masses in nuclear matter. 
Frank Klingl, Sungsik Kim, Su Houng Lee, Philippe Morath and Wolfram   Weise