On the universality of the x and A-dependence of the EMC effect and its relation to parton distributions in nuclei

It is shown that the latest results from the NMC (CERN) and E665 (Fermilab) groups on $F_2~A(x)/F_2~D(x)$ obtained in the shadowing region bring new evidence of the universal $A$ dependence of distortions made in a free-nucleon structure function by a nuclear medium. The observed universality implies that one can consider separately hard ($A\leq$ 4) and soft ($A>$ 4) parton distribution distortions. Soft distortions, which result in differencies between the deep-inelastic scattering cross-sections for nuclei with masses $A_1$, $A_2 \geq$ 4, can be explained as a consequence of the nuclear density variation, independent of $x$ in the range 0.001 $\leq x \leq$ 0.7. It is found that nuclear shadowing begins at $x_{\rm I}$ = 0.0615 $\pm$ 0.0024, independent of $A$, which is consistent with models that allow for three-parton recombination processes