NUCLEAR SHADOWING EFFECTS ON CHARMONIUM PRODUCTION IN QCD

It is shown that the effects on charmonium production of the modificat ion of nucleon parton distributions inside nuclei are similar in the f actorization approach based on nonrelativistic QCD and in the color ev aporation model. In the first model, a separate study of the color oct et and color singlet contributions to the yields of the various charmo nium states as well as the contributions of these states to the total J/Psi production is performed. It is found that a clear x(F) dependenc e of these contributions which, together with an overall normalization given by absorption of the pre-resonant c (c) over bar state inside t he nuclear medium, can reproduce experimental data for moderate x(F). However, at large x(F), there is a bigger suppression that cannot be f it by this overall normalization together with the shadowing of struct ure functions.