Diffraction at HERA, color opacity and nuclear shadowing in DIS off nuclei

The QCD factorization theorem for diffractive processes in DIS is used to d erive formulae for the leading twist contribution to the nuclear shadowing of parton distributions in the low thickness limit (due to the coherent pro jectile (photon) interactions with tare nucleons). Based on the current ana lyzes of diffraction at HERA we find that the average strength of the inter actions which govern diffraction in the gluon sector at x less than or equa l to 10(-3), Q(0) = 2GeV is similar to 50mb. This is three times larger tha n in the quark sector and suggests that applicability of DGLAP approximatio n requires significantly larger Q(0) in the gluon sector. We use this infor mation on diffraction to estimate the higher order shadowing terms due to t he photon interactions with N greater than or equal to 3 nucleons which are important for the scattering of heavy nuclei and to calculate nuclear shad owing and Q(2) dependence of gluon densities. For the heavy nuclei the amou nt of the gluon shadowing: G(A)(x, Q(0)(2))/AG(N) (x, Q(0)(2))\(x less than or equal to 10-3) similar to 0.25 - 0.4 is sensitive to the probability of the small size configurations within wave function of the gluon "partonome ter" at the Q(0) scale. At this scale for A similar to 200 the nonperturbat ive contribution to the gluon density is reduced by a factor of 4 - 5 at x less than or equal to 10(-3) unmasking PQCD physics in the gluon distributi on of heavy nuclei. We point out that the shadowing of this magnitude would strongly modify the first stage of the heavy ion collisions at the LHC ene rgies, and also would lead to large color opacity effects in eA collisions at HERA. energies. In particular, the leading twist contribution to the cro ss section of the coherent J/psi, production off A 2 greater than or equal to 12 nuclei at root s greater than or equal to 70 GeV is strongly reduced as compared to the naive color transparency expectations. The Gribov black body limit for F-2A(x, Q(2)) is extended to the case of the gluon distribut ions in nuclei and shown to be relevant for the HERA kinematics of eA colli sions. Properties of the final states are also briefly discussed.