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 F2a(x)F2d(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 ≤ 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 A1, A2 ≥ 4, can be explained as a consequence of the nuclear density variation, independent of x in the range 0.001 ≤ x ≤ 0.7. It is found that nuclear shadowing begins at x1 = 0.0615 ± 0.0024, independent of A, which is consistent with models that allow for three-parton recombination processes.