Overview

Nucleon spin structure has been of great interest since the CERN SMC experiment found that quarks carry only a small fraction of the nucleon spin. Since naively quarks are expected to carry most of the nucleon's spin, this observation became known as spin crisis. The availability of polarized beams and targets since the late 1970's has allowed the measurement of the nucleon spin structure functions, and furthermore checks of Quantum Chromodynamics (QCD), the theory of strong interactions, in its perturbative regime. Sum rules involving the spin structure of the nucleon offer an important opportunity to study QCD. It has recently been shown that the GDH sum rule and Bjorken sum rule are both special examples of a more general sum rule that applies to polarized electron scattering off nucleons. This generalized sum rule applies at all Q2. By studying the Q2 evolution of the extended GDH sum, one learns about the transition from quark-like behavior at high Q 2, larger than 1 GeV2, to hadronic-like behavior at low Q2, close to 0. The transition between these two regimes is far from understood; it is a key issue in our understanding of the strong interaction. During mid 2003, we measured inclusive polarized cross sections in the scattering of high energy polarized electrons off the Jefferson Lab Hall A polarized 3He target. A strained GaAs photocathode was utilized in the polarized electron gun to provide an electron beam with a polarization of 70%. The high density polarized 3He target, based on optical pumping of rubidium and spin echange, was used as an effective neutron target. The polarization of the target was maintained at about 40% at beam currents as high as 15 muA. The target polarization was measured by Nuclear Magnetic Resonance methods calibrated using two independent techniques: proton thermal polarization (water calibration) and Electron Paramagnetic Resonance (EPR). The relative uncertainty on the target polarization was better than 4%. The target was operated with both longitudinal and transverse target spin orientations. In Jlab Hall A, in order to reach very low Q2 in experiment E97-110, a new septum magnet was used to make very small scattering angle measurements with the Hall A right spectrometer. The left spectrometer was used without a septum to monitor an unpolarized carbon target, downstream of the polarized 3He target, to check for possible false asymmetries. A wide range of data were measured: beam energies from 1.15 to 4.40 GeV were used at scattering angles of 6 and 9 degrees. The invariant mass W ranged across the resonance region, from inelastic threhold to the beginning of the deep inleastic region (W = 2 GeV). From the measured spin-dependent cross sections we extract the 3He spin structure functions g1 and g2. Finally, we determined the extended GDH sum and other quantities for the Q2 range between 0.04 and 0.3 GeV 2. The range of data allows extrapolation to the real photon point, studying the convergence of the sum rule, determining the slope of GDH sum rule as Q2 → 0 to check Chiral Perturbation Theory, and overlap with the previous higher Q2 experiment E94-010.

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