Transverse dynamics of hard partons in nuclear media and the QCD dipole

We derive the non-abelian generalization of the Furry approximation whichdescribes the transverse dynamical evolution of a hard projectile parton insidea spatially extended colour target field. This provides a unified startingpoint for the target rest frame description of the nuclear dependence of alarge class of observables. For the case of the virtual gamma -> q qbarphotoabsorption cross section, we investigate in detail under which conditionsthe nuclear dependence encoded in the Furry wavefunctions can be parametrizedby a q-qbar QCD dipole cross section. The important condition is colourtriviality, i.e., the property that for arbitrary N-fold rescatteringcontributions the only non-vanishing colour trace is N_c C_F^N. We give proofsfor the colour triviality of the inelastic, diffractive and totalphotoabsorption cross section measured inclusively or with one jet resolved inthe final state. Also, we list examples for which colour interference effectsremain. Colour triviality allows us to write the DIS nuclear structure functionF_2 for small Bjorken x in terms of a path integral which describes thetransverse size evolution of the q-qbar pair in the nuclear colour field. Thisexpression reduces in an opacity expansion to the N=1 result of Nikolaev andZakharov, and in the eikonal approximation to the Glauber-type rescatteringformulas first derived by Mueller. In the harmonic oscillator approximation ofthe path integral, we quantify deviations from the eikonal limit. Their onsetis characterized by the scales L/l_f and E_t L which relate the longitudinalextension L of the nuclear target to the coherence length l_f and the totaltransverse energy E_t accumulated by the q-qbar-pair.