Colour coherent phenomena on nuclei and the QCD evolution equation

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Abstract
We review the phenomenon of colour coherence in quantum chromodynamics (QCD), its implications for hard and soft processes with nuclei, and its experimental manifestations. The relation of factorization theorems in QCD with colour coherence phenomena in deep inelastic scattering (DIS) and colour coherence phenomena in hard exclusive processes is emphasized. Analysing numerically the QCD evolution equation for conventional and skewed parton densities in nuclei, we study the onset of generalized colour transparency and nuclear shadowing of the sea quark and gluon distributions in nuclei as well as related phenomena. Novel results such as the dependence of the effective coherence length on Q2 and general trends of the QCD evolution are discussed. The limits of the applicability of the QCD evolution equation at small Bjorken x are estimated by comparing the inelastic quark-antiquark- and two gluon-nucleon (nucleus) cross sections, calculated within the DGLAP approximation, with the dynamical boundaries, which follow from the unitarity of the S-matrix for purely QCD interactions. We also demonstrate that principles of colour coherence play an important role in the processes of soft diffraction off nuclei.