The color dipole picture of low-x DIS

Abstract

Deep inelastic electron scattering (DIS) from nucleons at low values of the Bjorken variable $x \cong Q^2/W^2 \lsim ~ 0.1$ 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, $\sigma_{\gamma^*p} (W^2,Q^2) = \sigma_{\gamma^*p} (\eta (W^2,Q^2))$, with definite functional dependence on the low-x scaling variable $\eta (W^2,Q^2) \cong Q^2/\Lambda^2_{sat} (W^2)$ in the limits of $\eta (W^2,Q^2) \gg 1$ and $\eta (W^2,Q^2) \ll 1$, respectively. Consistency with the pQCD-improved parton model implies the definite value of $C_2 \cong 0.29$ for the exponent in the "saturation scale", $\Lambda^2_{sat} (W^2) \approx (W^2)^{C_2}$. The longitudinal-to-transverse ratio of the photoabsorption cross section at large $Q^2$ has the definite value of $R = 1/2 \rho$ with $\rho = 4/3$. For $W^2 \to \infty$ at any fixed $Q^2$, the photoabsorption cross section converges towards a $Q^2$-independent saturation limit that coincides with the cross section for $Q^2 = 0$ photoproduction.