# Spin Sum Rules: Transition from Quarks to Nucleons

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 g_{1} and g_{2} 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 Q^{2} indicates a smooth, yet dramatic, transition. They also show that the effective theory used at small Q^{2} (chiral perturbation theory) is not as reliable as first expected. At higher Q^{2}, the first order perturbative QCD calculation works down to surprisingly low Q^{2}. This suggests possible cancellations of higher-order effects such as quark-gluon and quark-quark correlations.

(See also entry on Color Polarizabilities)

References:

M. Amarian et al., Phys. Rev. Lett. 89 (2002) 242301

R. Fatemi et al., Phys.Rev.Lett. 91 (2003) 222002

M. Amarian et al., Phys.Rev.Lett. 92 (2004) 022301

A. Deur et al., Phys.Rev.Lett. 93 (2004) 212001