Scaling in $γ^* p$ total cross sections and the generalized vector dominance/color dipole picture

The scaling in $σ_{γ^*p}$ cross sections (for $Q^2/W^2 << 1$) in terms of the scaling variable $η= (Q^2 + m^2_0)/Λ^2 (W^2)$ is interpreted in the generalized vector dominance/color-dipole picture (GVD/CDP). The quantity $Λ^2 (W^2)$ is identified as the average gluon transverse momentum absorbed by the $q \bar q$ state, $<\vec l^{~2}> = (1/6) Λ^2 (W^2)$. At any $Q^2$, for $W^2 \to \infty$,the cross sections for virtual and real photons became universal, $σ_{γ^*p}(W^2,Q^2)/σ_{γp}(W^2) \to 1$.

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