Color Transparency and Saturation in QCD

Abstract: We review the theoretical interpretation of deep-inelastic electron-proton scattering at low values of the Bjorken variable $x \simeq Q^2/W^2 \lsim 0.1$. The process proceeds via the interaction of quark-antiquark $(q \bar q)$ color-dipole fluctuations of the (virtual) photon with the proton. In terms of the forward Compton scattering amplitude, two reaction channels contribute to the interaction of the $q \bar q$ color dipole with the gluon field in the proton. Dependent on the kinematics, there is either {\it color transparency}, corresponding to a cancellation of the amplitudes for the two reaction channels, or {\it saturation}, occuring when the process is dominated by a single interaction channel. The connection between the color-dipole picture and the pQCD improved parton model is elaborated upon.