Narrow Results

We review the theoretical interpretation of deep-inelastic electron–proton scattering at low values of the Bjorken variable x ≃Q²/W² ≲ 0.1. The process proceeds via the interaction of quark–antiquark 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 color dipole with the gluon field in the proton. Dependent on the kinematics, there is either color transparency, corresponding to a cancellation of the amplitudes for the two reaction channels, or saturation, occurring 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.

Based upon the color-dipole picture, we provide closed analytic expressions for the longitudinal and the transverse photoabsorption cross-sections at low values of the Bjorken variable of $x\lesssim 0.1$. We compare with the experimental data for the longitudinal-to-transverse ratio of the (virtual) photoabsorption cross-section and with our previous fit to the experimental data for the total photoabsorption cross-section. Scaling in terms of the low-$x$ scaling variable $\eta (W^2,Q^2)$ is analyzed in terms of the reduced cross-section of deep inelastic scattering.