Scaling and asymptotic behavior of the ${\gamma }^{\ast }p$ total cross section at low x

The scaling in ${\sigma }_{{\gamma }^{\ast }p}$ cross sections (for $Q^2\mathrm{/}{W}^2<<1$) in terms of the scaling variable $\eta =(Q^2+m_0^2)\mathrm{/}{\mathrm{\Lambda }}^2(W^2)$ is interpreted in the generalized vector dominance/color-dipole picture (GVD/CDP). The quantity ${\mathrm{\Lambda }}^2(W^2)$ is identified as the average gluon transverse momentum absorbed by the $q\bar{q}$ state, $<\vec l^{~2}> = (1/6) \Lambda^2 (W^2)$\mathrm{.}Atany$Q^2$,for$W^2 \to \infty$, the cross sections for virtual and real photons become universal, $\sigma_{\gamma^*p}(W^2,Q^2)/\sigma_{\gamma p}(W^2) \to 1$. The gluon density corresponding to the color-dipole cross section in the appropriate limit is found to be consistent with the results from QCD fits.