Spin Sum Rules: Transition from Quarks to Nucleons

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Q2 evolution of integrals of spin structure functions (left: proton, middle: neutron, right: proton-neutron difference). Enlarge

The connection between the fundamental quark-gluon and effective hadron descriptions of the strong interaction is a key question of nuclear physics studied at Jefferson Lab. Data obtained with JLab's multi-GeV polarized electron beam and polarized targets in Halls A and B on the spin-dependent g1 and g2 structure functions allow for precise extractions of the Bjorken and Gerasimov-Drell-Hearn integrals for the proton and neutron. These quantities are well suited to study the transition from quark-gluon to hadron degrees of freedom, as they can be compared to both fundamental and effective theory predictions.

The data evolution with Q2 indicates a smooth, yet dramatic, transition. They also show that the effective theory used at small Q2 (chiral perturbation theory) is not as reliable as first expected. At higher Q2, the first order perturbative QCD calculation works down to surprisingly low Q2. This suggests possible cancellations of higher-order effects such as quark-gluon and quark-quark correlations.

(See also entry on Color Polarizabilities)

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