Charmonium in Nuclear Matter and Chiral Symmetry

Changes in the masses of charmonium states in nuclear matter as a result of the modification of QCD vacuum in nuclear medium are studied in the perturbative QCD approach. For the leading-order effect due to change of gluon condensate, we use the leading-order QCD formula. The higher-twist effect that is related to the partial restoration of chiral symmetry, we use instead a hadronic model that includes the effect due to change of quark condensate in nuclear medium. It is found that although the mass of J/ψ decreases slightly in nuclear matter, those of ψ(3686) and ψ(3770) states are reduced appreciably. Experimental study of the mass shift of charmonium states in nuclear matter can thus provide valuable information on how QCD vacuum changes in nuclear medium.