New Results on Nucleon Spin Structure

Recent precision spin structure data from Jefferson Lab have significantly advanced our knowledge of nucleon structure in the valence quark (high-x) region and improved our understanding of higher-twist effects, spin sum rules and quark-hadron duality. First, results of a precision measurement of the neutron spin asymmetry, A 1 , in the high-x region are discussed. The new data shows clearly, for the first time, that A 1 becomes positive at high x. They provide crucial input for the global fits to world data to extract polarized parton distribution functions. Preliminary results on Ap1 and A d 1 in the high-x region have also become available. The up and down quark spin distributions in the nucleon were extracted. The results for ∆d/d disagree with the leading-order pQCD prediction assuming hadron helicity conservation. Then, results of a precision measurement of the g 2 structure function to study higher-twist effects are presented. The data show a clear deviation from the lead-twist contribution, indicating a significant higher-twist (twist-3 or higher) effect. The second moment of the spin structure functions and the twist-3 matrix element d 2 results were extracted at a high Q of 5 GeV from the measured A 2 in the high-x region in combination with existing world data and compared with a Lattice QCD calculation. Results for d 2 at low-to-intermediate Q from 0.1 to 0.9 GeV were also extracted from the JLab data. In the same Q range, the Q dependence of the moments of the nucleon spin structure functions was measured, providing a unique bridge linking the quark-gluon picture of the nucleon and the coherent hadronic picture. Sum rules and generalized forward spin polarizabilities were extracted and compared with Chiral Perturbation Theory calculations and phenomenological models. Finally, preliminary results on the resonance spin structure functions in the Q range from 1 to 4 GeV were presented, which, in combination with DIS data, will enable a detailed study of the quark-hadron duality in spin structure functions.