The energy dependence of the saturation scale in DIS at low x ∗

Consistency of the previously suggested color-dipole representation of deepinelastic scattering (DIS) and vector-meson production at low x with DGLAP evolution allows one to predict the exponent of the W 2 dependence of the saturation scale, Λsat(W ) ∼ (W )2 . One finds C 2 = 0.27 in agreement with the model-independent analysis of the experimental data from HERA on deepinelastic electron scattering. ∗ Supported by DFG-grant Schi 189/6-1 ∗∗ On leave of absence from Institute of Physics, Meiji Gakuin University, Yokohama 244, Japan The present paper is concerned with deep inelastic electron scattering (DIS) at low x ∼= Q/W 2 ≪ 1. In short, we analyse the consistency between DGLAP evolution of the nucleon structure function F2(x,Q ) and the color-dipole picture. We find that the exponent C 2 that in our formulation of the color-dipole approach determines the energy dependence of the total photoabsorption cross section at large Q, σγ∗p(W , Q) ∼ (W )2/Q, or, equivalently, the energy dependence of the “saturation scale” Λsat(W ) ∼ (W )2 , coincides with the result of previous fits to the experimental data, C 2 ∼= C experiment 2 . For x ∼= Q/W 2 ≪ 1, the photon-proton interaction is dominated by the interaction of the photon with the quark-antiquark sea in the proton. The proton structure function for x ≪ 1 only contains the flavor-singlet quark distributions, and their evolution in Q for Q ≥ Q0 [1] is in good approximation determined by the gluon structure function alone [2], ∂F2 (