Structure functions in nuclei: Quark clusters and size effects

Two of the most interesting ideas to explain the EMC effect, clusters of quarks and confinement size factors, are unable to fully explain the existing data. In this paper we argue that the existence of clusters with different numbers of quarks are in the origin of different size parameters controlling the lowQ ² limit in the QCD evolution and thus we combine the two ideas. As a result we obtain a model for structure functions with two basic effective parameters: the average numberi of nucleons per cluster and the corresponding size parameterR _i . We achieve good agreement with the existing data for a wide range ofQ ²(?2 GeV²) and for all values ofx. Simple parametrizations of the Sea, Gluon and Valence distributions, for pion, nucleon and nuclei, are given. Some applications of the model, in particular to Drell-Yan processes, are discussed.