Geometric Scaling from GLAP evolution

We show that the geometric scaling of the total virtual photon-proton cross section data can be explained using standard linear GLAP perturbative evolution with generic boundary conditions in a wide kinematic region. This allows us to single out the region where geometric scaling may provide evidence for parton saturation.Comment: Final version, published in Phys. Rev. Letters. References and minor clarifications added. 4 pages, 5 figures, LaTeX with REVTe

HERA data and DGLAP evolution: theory and phenomenology

We examine critically the evidence for deviations from next-to-leading order perturbative DGLAP evolution in HERA data. We briefly review the status of perturbative small-x resummation and of global determinations of parton distributions. We show that the geometric scaling properties of HERA data are consistent with DGLAP evolution, which is also strongly supported by the double asymptotic scaling properties of the data. However, backward--evolution of parton distributions into the low x, low Q^2 region does show evidence of deviations between the observed behaviour and the next-to-leading order predictions. These deviations cannot be explained by missing next-to-next-to-leading order perturbative terms, and are consistent with perturbative small-x resummation.Comment: Fig. 8 corrected. Published in NP