On the scaling properties of the total $\gamma^*\mathrm{p}$ cross section

Abstract

We perform a detailed analysis on the scaling properties of the total $\gamma^*\mathrm{p}$ cross section, $\sigma_{\gamma^*\mathrm{p}}$. We write the cross section as a product of two functions $W$ and $V$ representing, respectively, the dynamical degrees of freedom and the contribution from the valence partons. Analyzing data from HERA and fixed target experiments, we find that $V$ is independent of $Q^2$ and concentrated at large $x$, while $W$ carries all the information on the $Q^2$ evolution of $\gamma^*\mathrm{p}$. We define the reduced cross section $\tilde{\sigma}_{\gamma^*\mathrm{p}} \equiv W=\sigma_{\gamma^*\mathrm{p}}/V$, and show that it is very close to a generalized homogeneous function. This property gives rise to geometric scaling for $\tilde{\sigma}_{\gamma^*\mathrm{p}}$ and it also explains the known geometric scaling of $\sigma_{\gamma^*\mathrm{p}}$ at low $x$. As a consequence of our {\em Ansatz}, we also obtain a compact parameterization of $\sigma_{\gamma^*\mathrm{p}}$ describing all data above $Q^2=1$~GeV$^2$.