On the $Q^2$ dependence of nuclear structure functions

The recent high statistics NMC data on the Tin to Carbon structure function ratio seems to indicate, for the first time, a significant $Q^2$ dependence, especially at small values of Bjorken $x$, $x < 0.05$, and $Q^2 > 1 GeV^2$. Pure leading twist, perturbative QCD--based predictions, which are consistent with the free nucleon data, yield a fairly flat ratio with little or no $Q^2$ dependence. In view of this seeming contradiction, we re-examine the applicability of such a perturbative model to nuclear structure functions in such a kinematical regime. We find that the model is consistent with all data, within experimental errors, without any need for introducing additional higher twist contributions. The model correctly reproduces the $Q^2$ dependence of the Carbon structure function as well. We also critically examine the $Q^2$ dependence of the corresponding spin dependent structure functions.