Spin structure of helium-3 and the neutron at low Q2: A measurement of the extended GDH integral and the Burkhardt-Cottingham sum rule.

摘要

Experiment 94-010 investigated the spin structure of the neutron and 3He at Jefferson Laboratory in Newport News, VA. Longitudinally polarized electrons of incident energy from 0.9 GeV to 5.0 GeV were scattered from a high pressure polarized 3He target in experimental Hall A. 3He was used as an effective neutron target, taking advantage of the approximate cancellation of the two spin-paired protons in the 3He ground state. Longitudinal and transverse target polarizations were maintained, allowing a precision determination of both spin structure functions g1(x,Q2) and g2(x,Q2). The structure functions were extracted from an inclusive polarized cross section measurement at low momentum transfer (0.1 Q2 0.9 GeV2) in the threshold, quasielastic, and resonance regions.; The goal of this experiment has been to test our grasp of Quantum Chromodynamics (QCD), the fundamental theory of the strong interaction, in the non-perturbative region. QCD has been verified impressively in the high energy region of asymptotic freedom, that is, when the magnitude of the force between quarks is relatively small. However, experimentally verifiable predictions at low Q 2 have been harder to achieve due to the complexity of the interactions between constituents. Definite predictions do exist however, for the first (and higher) moments of the spin structure functions in the form of the various QCD sum rules. The Burkhardt-Cottingham sum rule is a Q 2-dependent relation which implies the vanishing of the first moment of the g2 structure function, while the Bjorken and extended GDH sum rules follow directly from QCD and govern the behaviour of the g1 structure function. These integrals are investigated for a 3He (neutron) target at low momentum transfer.