The generalized vector dominance/colour-dipole picture of deep-inelastic scattering at low x

We give a detailed account of the recently formulated generalized vector dominance/colour-dipole picture (GVD/CDP) of deep-inelastic scattering at low $x\cong Q^2/W^2$, including photoproduction. The approach, based on $\gamma^*(q \bar q)$ transitions, $q \bar q$ propagation and diffractive $(q \bar q)p$ scattering via the generic structure of the two-gluon exchange, provides a unique and quantitatively successful theory for the $\gamma^* p$ total cross section, $\sigma_{\gamma^* p} (W^2,Q^2)$, at low $x$. The GVD/CDP is shown to imply the empirical low-$x$ scaling law, $\sigma_{\gamma^* p} (W^2,Q^2)=\sigma_{\gamma^* p} (\eta)$ with $\eta=(Q^2+m_0^2)/\Lambda^2(W^2)$, that was established by a model-independent analysis of the experimental data.