Helicity dependent parton distributions

The helicity dependent parton distributions describe the number density of partons with given longitudinal momentum x and given polarization in a hadron polarized longitudinally with respect to its motion. After the discovery , more than 70 years ago , that the proton is not elementary , the observation of Bjorken scaling in the late 1960s lead to the idea of hadrons containing almost pointlike constituents , the partons. Since then , Deep Inelastic Scattering (DIS) has played a crucial role in our understanding of hadron structure . Through DIS experiments it has been possible to link the partons to the quarks , and to unveil the presence of other pointlike constituents , the gluons , which lead into a dynamical theory of quarks and gluons-quantum chromodynamics (QCD). Polarized DIS , i. e. the collision of a longitudinally polarized lepton beam on a polarized target (either longitudinally or transversely polarized) , provides a complementary information regarding the structure of the nucleon. Whereas ordinary DIS probes simply the number density of partons with a fraction x of the momentum of the parent hadron , polarized DIS can partly answer the question as to the number density of partons with given x and given spin polarization in a hadron of definite polarization , either parallel or transverse with respect to the motion of the hadron. In here , the phenomenology associated with DIS off longitudinally polarized targets , and the present theoretical understanding of the dynamics involved , are described .