Deep-inelastic scattering and Drell-Yan process for the 3D structure of the nucleon

An overview of the study of the 3D structure of the nucleon is described. Both the longitudinal and transverse structures of the nucleon have been investigated with various experimental methods. The longitudinal structure or helicity distribution was studied first as the cross section of polarized deep-inelastic scattering was formulated with quark-parton model. The result reported by EMC in the papers in 1988 and 1989 showed that the contribution of spin of quarks and anti-quarks to the proton spin is only (12 +-  9 +- 14)%. The result was confirmed by the experiments in 1990's and later. Spin is one of the quantum numbers of the nucleon. The concept of 'valence quark' was questioned because the valence quarks were supposed to determine all the quantum numbers of the nucleon. Sea quarks are quarks and anti-quarks generated by the gluon dissociation inside the nucleon. Those sea quarks could also contribute to the nucleon spin. Gluon spin contribution was studied by proton-proton collisions and deep-inelastic scattering.

The transverse-momentum-dependent parton distributions (TMDs) and the orbital angular momenta of partons are another subjects under study. The single-spin asymmetry in meson productions was observed with increasing center-of-mass energy up to RHIC. Sivers- and Collins-asymmetries were separately identified by sin( phi - phi_S ) and sin( phi + phi_S ) modulations of the azimuthal distributions by HERMES in 2005. The coincidence measurement of the scattered lepton and produced hadrons turned out to be the effective method. Exclusive photon and meson productions were measured for generalized parton distributions. The deep-inelastic scattering is sensitive to the sum of parton distributions of quarks and anti-quarks while the Drell-Yan process is sensitive to the products of the two. The properties of anti-quarks can be studied with a combination of these two complementary approaches. The evolution and perspective of the study with these experimental methods are discussed.