Partonic structure of the Nucleon in QCD and Nuclear Physics: new developments from old ideas

Abstract

The nucleon is an ideal laboratory to solve QCD in the nonperturbative regime. There are several experimental observations that still lack a rigorous interpretation; they involve the nucleon as a (polarized) target as well as a beam (in collisions and Drell-Yan processes). These data look like big azimuthal and spin asymmetries, related to the transverse polarization and momentum of the nucleon and/or the final detected particles. They suggest internal reaction mechanisms that are suppressed in collinear perturbative QCD but that are "natural" in Nuclear Physics: quark helicity flips, residual final state interactions, etc.. In my talk, I will give a brief survey of the main results and I will flash the most recent developments and measurements.

Comments: to appear in the proceedings of the X Convegno su problemi di Fisica Nucleare e Teorica, Cortona (Italy), 6-9 Oct. 2004; ws-proc9x6.cls included