Parton Densities in a Nucleus at Small-x: Origin of Nuclear Shadowing Effect (with Z. Huang and H. J. Lu), Nuclear Physics A637, 79 (1998); Nuclear Physics A638, 531 (1998).

We investigate the nuclear shadowing mechanism in the context of perturbative QCD and the Glauber-Gribov multiple scattering model. Using recent HERA data on nucleon structure function at small $x$, we put stringent constrains on the nucleon gluon density in the double-logarithm approximation. We suggest that the scaling violation of the nucleon structure function in the region of small-$x$ and semihard scale $Q^2$ can be reliably described by perturbative QCD which is a central key to the understanding of the scale dependence of the nuclear shadowing effect. Our results indicate that while the shadowing of the quark density arises from an interplay between ``soft'' and semihard QCD processes, while gluon shadowing is largely driven by a perturbative shadowing mechanism. We demonstrate that the gluon shadowing is a robust phenomenon at large $Q^2$ and can be unambiguously predicted by perturbative QCD.