Parton distributions at low x and gluon- and quark average multiplicities

Abstract

We shown the general approach for Q 2 evolution of parton densities and fragmentation functions at low x based on the diagonalization. The diagonalization leads to the two components in the Q 2 evolution, each of which contains a nonperturbative parameter. The values of the parameters can be found by fits of the experimental data for the deep-inelastic scattering structure function F 2 and for average jet multiplicities. One of the components contains the all large logarithms ln(1/x) and produce the basic contribution at small x region. The second one is regular at low x but its contribution is very important to have a good agreement with experimental data.Recently, we proposed a new formalism [28,22,23] that solves the problem of the apparent good convergence of the perturbative series and does not require any ad-hoc offset, once the effects due to the mixing between quarks and gluons are fully included. Our result is a generalization of the result obtained in Ref. [25]. In our new approach, the nonperturbative informations to the