The Status of the Polarized Parton Densities

Deeply inelastic scattering provides a clean way to extract the parton densities of nucleons. After the initial observation that the nucleon spin is not formed by the quarks dominantly [1], detailed measurements of the polarized structure functions followed during the last 20 years. The central question concerns now the parton distribution functions and their scale evolution rather than just their first moment. Since the nucleon spin receives also contributions from the angular momentum of the quarks and gluons, these degrees of freedom have also to be studied. This requires the analysis of non–forward scattering cross sections. In inclusive deep–inelastic scattering the sensitivity to resolve the different sea–quark contributions is rather limited. One way to extract this information consists in measuring semi–inclusive processes [2]. A central question concerns the polarized gluon distribution, which can be determined from the scaling violations of g1(x,Q ), deep–inelastic heavy flavor production, and hard processes measured at hadron colliders. The inclusive and semi–inclusive hard processes in polarized scattering provide an important laboratory to test QCD. More than exploring the level of twist–2, which has been investigated in some detail already, one may probe the twist–3 contributions in various transverse spin processes. In the following we survey the theoretical status of inclusive polarized deeply inelastic scattering and the status of polarized parton densities, cf. also [3]. We close with an outlook on investigations required in the future.