Studies of the 3D structure of the proton at Jlab
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tudies of the 3D Structure of the Proton at Jlab1 H. Avakian Jefferson Lab, 12000 Jefferson Ave., Newport News, VA 23606 Abstract—In recent years parton distributions, describing longitudinal momentum, helicity and transversity distributions of quarks and gluons, have been generalized to account also for transverse degrees of freedom. Two new sets of more general distributions, Transverse Momentum Distributions (TMDs) and Generalized Parton Distributions (GPDs) were introduced to describe transverse momentum and spatial distributions of partons. Great progress has been made since then in measurements of different Single Spin Asymmetries (SSAs) in semiinclusive and hard exclusive processes, providing access to TMDs and GPDs, respectively. Studies of TMDs and GPDs are also among the main driving forces of the JLab 12 GeV upgrade project. DOI: 10.1134/S1063779614010092 1
1. INTRODUCTION
The orbital momentum of partons has been of cen tral interest since the SLAC and EMC measurements implied that the helicity of the constituent quarks account for only a fraction of the nucleon spin. Single spin asymmetries (SSA) in azimuthal distributions of finalstate particles in semiinclusive deep inelastic scattering (DIS) play a crucial role in the study of transverse momentum distributions of quarks in the nucleon and provide access to the orbital angular momentum of quarks. In recent years, measurements of azimuthal moments of polarized hadronic cross sections in hard processes, and in particular the SSAs, have emerged as powerful tools to probe nucleon structure through their sensitivity to Generalized Par ton Distributions (GPDs) and Transverse Momentum Dependent parton distribution functions (TMDs) in hard exclusive and semiinclusive production of final states particles, respectively (see table). The first unambiguous measurements of single spin phenom ena in SIDIS, which triggered important theoretical developments, were the sizable longitudinal target spin sin φ asymmetries ( A UL ) observed at HERMES [1, 2]. Measurements of nonzero SSAs in electroproduction attracted a huge amount of theoretical and experi mental interest, and many experiments worldwide are currently trying to pin down various effects related to nucleon structure through semiinclusive deepinelas tic scattering (HERMES at DESY, COMPASS at CERN, Jefferson Lab), polarized protonproton col lisions (PHENIX, STAR and BRAHMS at RHIC), and electronpositron annihilation (Belle at KEK). Two fundamental QCD mechanisms giving rise to sin gle spin asymmetries were identified. First, the Collins mechanism [3–5], where the asymmetry is generated in the fragmentation of transversely polarized quarks, 1 The article is published in the original.
and second, the Sivers mechanism [6–9], where the asymmetry is generated by final state interactions at the distributionfunction level. Studies of SSAs are currently driving the upgrades of several existing facil ities (JLab and RHIC) and the design and construc tion of new facilities worldwide (EIC, GSI, and JP
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