The $γ^* \lowercase{p}$ total cross section at low x

Abstract

The scaling in ${\sigma }_{{\gamma }^{\ast }p}(W^2,Q^2)$ cross sections (for $Q^2/W^2<<1$) in terms of the scaling variable $\eta =(Q^2+m_0^2)/{\Lambda }^2(W^2)$ is interpreted in the generalized vector dominance/color-dipole picture (GVD/CDP). The quantity ${\Lambda }^2(W^2)$ is identified as the average gluon transverse momentum absorbed by the $q\bar{q}$ state, $<{\vec{l}}_{\perp }^2>=(1/6){\Lambda }^2(W^2)$. At any $Q^2$, for $W^2\to \infty$, the cross sections for virtual and real photons become universal,${\sigma }_{{\gamma }^{\ast }p}(W^2,Q^2)/{\sigma }_{\gamma p}(W^2)\to 1$. 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.

8 pages, Latex using psfig, 5 figures Invited Talk presented at the XXXVth Internat. School of Theoretical Physics, Ustron, Poland, Sept. 10-16, 2001 and at the Internat. Conf. "New Trends in High-Energy Physics", Yalta, Crimea, Ukraine, Sept. 22-29, 2001