Nuclear suppression of hadroproduction.

We demonstrate that the main part of the observed nuclear suppression of dilepton and charmonium production, especially the x-dependence of the suppression, can be explained by the absorption of fast initial partons. We assume the absorption to be x 1-dependent and determine the empirical form of this dependence. Several reactions at different energies are described with the same parametrization of the absorption cross section. The factorization theorem constraints and some alternative models are discussed. The obtained description of data allows to make conclusions about the origin of nuclear effects in hadro-and electroproduction. In particular, it is concluded that the parton recombination and the excess pion contribution are unimportant in nuclei and an indication of excess gluons at x 2 ∼ 0.1 is observed.