Spin Effects in Lepton-nucleon Scattering: a Theoretical Overview

The physics of polarized nucleons has received increasing interest in the past few years, as we had the opportunity to see during the Workshop. Experimental progress in this field has been impressive, and prospect for the next future are even more interesting. A description of the present status of the field on the experimental side is given in another chapter of these Proceedings; 1 here I will concentrate on the theoretical aspects. The most important tool for studying the structure of polarized nucleons is polarized deep-inelastic scattering, and most of the work presented in our Workshop was devoted to the study of polarized structure functions; I conclude this introduction recalling a few basics concepts in this field. In Section 2, I will discuss the problem of scaling violation in polarized deep inelastic scattering, in the context of perturbation theory. In Section 3 I will review the theoretical status of the small x behaviour of polarized structure functions, and in Section 4 I will discuss the extraction of polarized parton densities. Finally, in Section 5 I present my conclusions. The antisymmetric part of the hadronic tensor, relevant for polarized deep inelastic lepton-nucleon scattering, is usually decomposed as