New Results on Nucleon Spin Structure

Abstract

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, $A_1^n$, in the high-$x$ region are discussed. The new data shows that $A_1^n$ becomes positive at high $x$. They provide crucial input for the global fits to world data to extract polarized parton distribution functions. The up and down quark spin distributions in the nucleon were extracted. The results for $\Delta d/d$ disagree with the leading-order pQCD prediction assuming hadron helicity conservation. Then, results of a precision measurement of the $g_2^n$ structure function to study higher-twist effects are presented. The data show a clear deviation from the lead-twist contribution. The second moment of the spin structure functions and the twist-3 matrix element $d_2^n$ results were extracted at a high $Q2$ of 5 GeV$^2$. Results for $d_2^n$ at low-to-intermediate $Q2$ from 0.1 to 0.9 GeV$^2$ were also extracted from the JLab data. In the same $Q2$ range, the $Q2$ 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. Finally, preliminary results were presented on the resonance spin structure functions in the $Q2$ range from 1 to 4 GeV$^2$ to study the quark-hadron duality.