The Spin Structure of the Nucleon
Figure 2: Large-x JLab data on quark polarizations. The solid lines include quark orbital anglar momentum while the dashed lines do not.
Nucleon spin is being extensively studied at JLab [1-4], addressing fundamental questions unanswered by earlier generations of experiments. While previous experiments made important contributions to our understanding of the nucleon spin (for example that quark spins alone cannot explain it), crucial questions such as the role of gluon polarization and quark orbital angular momentum remained unresolved. With its unique capabilities, JLab has investigated these questions with high-precision measurements. Our knowledge of the gluon polarization ? significantly improved after the JLab polarized structure function data were included in the world data set and reanalyzed recently in Ref. , see Fig. 1. Furthermore, the effect of the quark orbital momentum ?, which has been difficult to measure in experiments, can be seen in the large-x data in Fig. 2, where the predictions based on perturbative QCD (dashed curves) disagree with JLab large-x measurements if quark orbital momenta are neglected. The perturbative QCD results and data are reconciled only after quark orbital momentum components are added to the nucleon wave function , (solid line in Fig. 2).
 For a review of older JLab data, see e.g. J.P. Chen, A. Deur et Z.-E. Meziani, Mod. Phys. Lett. A20 (2005) 2745
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