Colour coherent phenomena on nuclei and the QCD evolution equation

We review the phenomenon of colour coherence in quantum chromodynamics (QCD ), its implications for hard and soft processes with nuclei, and its experi mental manifestations. The relation of factorization theorems in QCD with c olour coherence phenomena in deep inelastic scattering (DIS) and colour coh erence phenomena in hard exclusive processes is emphasized. Analysing numer ically the QCD evolution equation for conventional and skewed parton densit ies in nuclei, we study the onset of generalized colour transparency and nu clear 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 Q(2) and general trends of the QCD evolution are discu ssed. The limits of the applicability of the QCD evolution equation at smal l Bjorken x are estimated by comparing the inelastic quark-antiquark- and t wo gluon-nucleon (nucleus) cross sections, calculated within the DGLAP appr oximation, with the dynamical boundaries, which follow from the unitarity o f the S-matrix for purely QCD interactions. We also demonstrate that princi ples of colour coherence play an important role in the processes of soft di ffraction off nuclei.