Scaling in ${\gamma }^{\ast }p$ total cross sections, saturation and the gluon density

Abstract

Including the new HERA data, the ${\gamma }^{\ast }p$ total cross section is analysed in the generalized vector dominance/colour-dipole picture (GVD/CDP) that contains scaling in $\eta =(Q^2+m_0^2)/{\Lambda }^2(W^2)$, where ${\Lambda }^2(W^2)$ is an increasing function of $W^2$. At any $Q^2$, for $W^2\to \infty$, the cross sections for virtual and real photons become identical, ${\sigma }_{{\gamma }^{\ast }p}(W^2,Q^2)/{\sigma }_{\gamma p}(W^2)\to 1$. The gluon density deduced from the colour-dipole cross section fulfills the leading order DGLAP relationship. Evolution à la DGLAP breaks down for $\eta \text{lsim}0.1$.

Comments: 15 pages, Latex using epsfig, 6 figures using 8 ps files