We have today a picture of a nucleon composed of 3 valence quarks, plus a large number of quarks-antiquarks pairs and gluons, which are continuously created and anihilated, all interacting together. Many questions are still open when trying to describe this complex structure, and experiments are mounted to answer these questions. We will focus on dilepton pair production experiments, and in particular two kind of reactions, which we will discuss.
The first kind concerns near-future experiments at the Jefferson Laboratory, USA. It is the exclusive production (where all final products are detected) of a lepton pair, from an electron or photon beam colliding with a nucleon. The exclusive caracter of these reactions allows them to be sensitive to both the transverse distribution of partons and their longitudinal momentum. Generalized Parton Distributions which describe the non-perturbative part of the process can be interpreted for instance to perform nucleon tomography.
The second kind concerns in particular the PHENIX experiment at the Brookhaven National Laboratory, USA. It is the inclusive production (where only selected products are detected) of a lepton pair, from the collision of two hadrons (protons, or proton-ion in the PHENIX case). The well known Drell-Yan reaction is sensitive to intrisic transverse parton momentum and their longitudinal momentum. In the case of a transversaly polarized beam, the Sivers function can be measured which describes the distribution of quarks in a transversaly polarized nucleon. Theory predicts a sensitivity to gluons radiated from quarks prior to the collision.
