Diffraction at HERA, Color Opacity and Nuclear Shadowing

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The QCD factorization theorem for diffractive processes in DIS is used to derive formulae for the leading twist contribution to the nuclear shadowing of parton distributions in the low thickness limit. Based on the current analyzes of diffraction at HERA we find that the average strength of the interactions which govern diffraction in the gluon sector at x ≤ 10⁻³, Q₀=2 GeV is 50 mb. This is 3 times larger than in the quark sector and suggests that applicability of DGLAP approximation requires significantly larger Q₀ in the gluon sector. We use this information to estimate quark and gluon shadowing for heavy nuclei and to calculate itsQ² depen- dence. For A 200 the amount of the gluon shadowing: Gₐ/AGₙ 0.25-0.4 at x<10⁻³,Q=2GeV is sensitive to the probability of the small size configurations within wave function of the gluon "partonometer" at the Q₀ scale. At this scale for A 200 the nonperturbative contribution to the gluon density is reduced by a factor of 4-5 at x ≤ 10⁻³ unmasking PQCD physics in the gluon distribution of heavy nuclei. Such shadowing would strongly modify the first stage of the heavy ion collisions at LHC, and would lead to large color opacity effects in eA collisions at x<0.001. The leading twist contribution to the cross section of the coherent J/ψ production off A≥ 12 nuclei at √ s ≥ 70 GeV is strongly reduced as compared to the naive color transparency expectations. The Gribov black body limit for F₂ₐ(x,Q²) is extended to the case of the gluon distributions in nuclei and shown to be relevant for the HERA kinematics of eA collisions. Properties of the final states are also briefly discussed.