The color dipole picture of low-x DIS

Deep inelastic electron scattering (DIS) from nucleons at low values of the Bjorken variable x≅Q2/W2≲0.1 proceeds via fluctuations of the photon into quark-antiquark dipole states that subsequently interact with the gluon field in the nucleon. Dependent on the interaction energy, W, the color-gauge-invariant dipole interaction with the gluon field in the nucleon, for any fixed dipole size, contains the limits of i) color transparency and ii) saturation, where “saturation” stands for the approach to a hadronlike dipole-proton interaction cross section. All essential features of the experimental results on low-x DIS, as a consequence of the color-gauge-invariant dipole interaction follow model independently i.e. without specific ansatz for the dipole cross section. The model-independent results in particular include the low-x scaling behavior of the photoabsorption cross section, σγ⁎p(W2,Q2)=σγ⁎p(η(W2,Q2)), with definite functional dependence on the low-x scaling variable in the limits of η(W2,Q2)≫1 and η(W2,Q2)≪1, respectively. Consistency with the pQCD-improved parton model implies the definite value of C2≅0.29 for the exponent in the “saturation scale”, . The longitudinal-to-transverse ratio of the photoabsorption cross section at large Q2 has the definite value of R=1/2ρ with ρ=4/3. For W2→∞ at any fixed Q2, the photoabsorption cross section converges towards a Q2-independent saturation limit that coincides with the cross section for Q2=0 photoproduction. In terms of the underlying gluon distribution, the transition from the region of validity of the pQCD-improved parton model at η(W2,Q2)>1 to the saturation region of η(W2,Q2)<1 corresponds to a transition from (approximate) proportionality of the proton structure function to the gluon distribution to a logarithmic dependence on the gluon distribution function. Our specific ansatz for the dipole cross section that interpolates between the limits of η(W2,Q2)≫1 and η(W2,Q2)≪1 describes the experimental data for the proton structure function in the full range of .