Small X Behaviour of Parton Distributions

The measurements of the deep-inelastic scattering structure function F2 in HERA 1 have permitted the access to a very interesting kinematical range for testing the theoretical ideas on the behavior of quarks and gluons carrying a very low fraction of momentum of the proton, the so-called small x region. In this limit one expects that non-perturbative effects may give essential contributions. However, the resonable agreement between HERA data and the NLO approximation of perturbative QCD that has been observed for Q > 1GeV (see the recent review in ) indicates that perturbative QCD could describe the evolution of structure functions up to very low Q values, traditionally explained by soft processes. Here we illustrate the results obtained recently in . These results are the extension to the NLO QCD approximation of previous LO studies . The main ingredients are: 1. Both, the gluon and quark singlet densities are presented in terms of two components (+ and −) which are obtained from the analytical Q dependent expressions of the corresponding (+ and −) parton distributions moments. 2. The − component is constant at small x, whereas the + component grows at Q ≥ Q20 as