Perturbative QCD Evolution and Color Dipole Picture

Abstract

The proton structure function in the diffraction region of small Bjorken-$x$ and $10{\mathrm{GeV}}^2\le Q^2\le 100{\mathrm{GeV}}^2$ behaves as $F_2(x,Q^2)=F_2(W^2)=f_0\cdot (W^2{)}^{C_2}$, where $x=Q^2/W^2$. The exponent $C_2$ of the ${\gamma }^{\ast }p$ center-of-mass energy squared, $W^2$, is predicted from evolution of the flavor-singlet quark distribution, $C_2=0.29$, and the only free parameter, the normalization $f_0=0.063$, is fitted. The evolution of the gluon density multiplied by ${\alpha }_s(Q^2)$ is dentical to the evolution of the flavor-singlet quark density. This simple picture is at variance with the standard approach to evolution based on the coupled equations of flavor-singlet and gluon density.

Comments: 11 pages, 4 figures