Abstract
Parton distribution functions, describing longitudinal momentum, helicity and transversity distributions of quarks and gluons, have been recently generalized to account also for transverse degrees of freedom. Two new sets of more general distributions, Transverse Momentum Distributions and Generalized Parton Distributions, were introduced to describe transverse momentum and space distributions of partons. Great progress has been made since then in measurements of different Single Spin Asymmetries (SSAs) in semi-inclusive and hard exclusive processes providing access to TMDs and GPDs, respectively. Facilities world-wide involved in studies of the 3D structure of nucleon include HERMES, COMPASS, BELLE, BaBar, Halls A, B, and C at JLab, and PHENIX and STAR at RHIC (BNL). TMD studies in the Drell-Yan process are also becoming an important part of the program of hadron scattering experiments. Studies of TMDs are also among the main driving forces of the JLab 12-GeV upgrade project, several of the forward upgrade proposals of STAR and PHENIX at RHIC, and future facilities, such as the Electron Ion Collider (EIC), FAIR in Germany, and NICA in Russia. In this contribution we present an overview of the latest developments in studies of parton distributions and discuss newly released results, ongoing activities, as well as some future measurements.
Highlights
Since the late 60s many theoretical and experimental groups dedicated their time to study the quarkgluon structure of nucleons and nuclei in terms of collinear ( “integrated”) parton distributions (PDFs) of hadrons
Transverse space distributions of partons, encoded in Generalized Parton Distributions (GPDs), and transverse momentum dependent distributions, encoded in Transverse Momentum Distributions (TMDs), have been widely recognized as key objectives of the JLab 12-GeV upgrade [1] and the polarized pp program at RHIC [2] as well as a driving force behind the construction of the Electron Ion Collider (EIC) [3]
Due to the dominance of gluon induced hard scattering processes at RHIC, longitudinally polarized proton-proton collisions studied by PHENIX and STAR are currently the best source of information on the gluon polarization, ∆g
Summary
Since the late 60s many theoretical and experimental groups dedicated their time to study the quarkgluon structure of nucleons and nuclei in terms of collinear ( “integrated”) parton distributions (PDFs) of hadrons. TMD and GPD distributions (see Table 1) describe partons with certain polarizations in nucleons in independence of the polarization state The gauge invariant interpretation of ∆g as the gluon spin contribution has been identified It has been shown, that to access the canonical orbital angular momentum one has to extract experimentally either the phase space Wigner distribution [6] or particular twist-3 distributions [5]. Hard exclusive production of light mesons allows to filter different GPDs by different final state hadrons [8, 9] In addition they provide a unique possibility to access “transversity” or chiralodd GPDs through measurements of different azimuthal moments of the cross section. In the handbag approach developed recently by Liuti and Goldstein [10] and Kroll and Goloskokov [11, 12] the helicity amplitudes depend on hard partonic subprocess and the GPD and the beam spin asymmetries of exclusive pions combined with other spin and azimuthal asymmetries, which can provide a unique possibility to access the elusive transversity GPDs
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