ar X iv : h ep - p h / 98 12 32 2 v 1 1 0 D ec 1 99 8 Diffraction at HERA , Color Opacity and Nuclear Shadowing

We calculate nuclear shadowing and Q 2 dependence of gluon densities within the framework of the Gribov theory of nuclear shadowing using as input recent HERA data on diffractive dijet and charm production in deep inelastic ep scattering. We explain that in the case of the deuteron and of the light nuclei (A ≤ 12) theoretical uncertainties in the calculations of shadowing are small. For the heavy nuclei the amount of shadowing is sensitive to the probability of small size configurations within wave function of the virtual photon, of the gluon partonometer at the normalization scale Q = 2GeV which violate the Regge pole factorization. At this scale for A ∼ 200 the nonperturbative contribution to the gluon densities is reduced by a factor of 4–5 at x ≤ 10 −3. unmasking PQCD physics in the gluon distribution of nuclei. We point out the shadowing of this magnitude would strongly modify the first stage of the heavy ion collisions at the LHC energies , and also lead to a large color opacity effects in eA collisions at HERA energies. In particular, the leading twist contribution to cross section of * On leave of absence from PSU.