Masses ofJ/ψandηcin the Nuclear Medium: QCD Sum Rule Approach

We investigate the masses of the lowest $c\overline{c}$ 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.