Connection between Diffraction and Small-x

Let us dream of e-Uranium DIS at THERA at Q ∼ 10 GeV and x ∼ 10. Violent DIS is associated with complete destruction of the target, deposition of a mere dozen MeV energy breaks the uranium nucleus entirely, yet a rigorous prediction from unitarity is that diffractive DIS eU → eXU with the target nucleus emerging intact in the ground state will make≈ 50% of total DIS ! By the Glauber-Gribov theory, the abundance of diffraction is closely related to nuclear shadowing (NS) in DIS. In 1974 Nikolaev and Zakharov reinterpreted NS in terms of the saturation of nuclear parton densities . More recently, the NZ picture of saturation has been addressed to within QCD, see Iancu’s talk at this Symposium . If correct, this QCD approach must inevitably lead to 50 % diffractive DIS. To summarize, diffractive DIS is a key to nuclear parton densities and QCD predictions for the initial state in ultrarelativistic nuclear collisions. Because at HERA the rate of diffractive DIS is mere 10%, saturation effects are all but marginal, see 4 and talks 5 at this Symposium .