Hadronic Physics Group
at the   Physics Department   of the   College of William and Mary
Nucleon Spin Structure using Polarized 3He

The ability to optically-pump 3He provides the opportunity to create a convenient approximation of a polarized neutron target (since to a high degree, the polarization of the 3He arises solely due to the neutron). The advent of high-density, high-polarization 3He targets opens up many possibilities in the study of the spin structure of nucleons and nuclei.

Led by Averett, a polarized 3He target lab (more details can be found here) has been developed here at W & M, in conjunction with Jefferson Lab, to produce and test the highly specialized glass target cells that are used in these targets.

A program of measurements in Hall A at JLab using these targets is based on inclusive scattering of longitudinally polarized electrons from 3He nuclei whose spins are oriented parallel or perpendicular to the incident electron direction. By flipping the helicity of the incoming electron, inclusive scattering asymmetries and cross sections can be measured using each of the two Hall A spectrometers independently.

These experiments include:

g2 Structure Function and Higher-Twist Effects    JLab Experiment 97-103

The spin structure function g2 is important when the target spin is perpendicular to the electron spin, and unlike g1, has no simple interpretation in the quark-parton model. Instead, it is uniquely sensitive to non-perturbative QCD effects such as quark-gluon interactions, in particular to `higher-twist' contributions where the incident photon interacts with a quark that is simultaneously exchanging a gluon with another quark.

Averett, along with W. Korsch (U. Kentucky) is co-spokesperson for this experiment, which measures g2 for the neutron at 5 points with Bjorken x = 0.17 in the range 0.6 < Q2 < 1.4 GeV2 by scattering polarized electrons from a transversely polarized 3He target. The experiment will achieve statistical errors which are roughly two orders of magnitude smaller than existing data in this kinematic range.

This experiment took data in the summer of 2001. Data analysis is well underway, and Kevin Kramer will base his PhD thesis on these data.
A1n : neutron spin structure at large x    JLab Experiment 99-117

Experiment E99-117 is aimed at measuring the neutron spin asymmetry A1n at three values of Bjorken x (0.33, 0.47 and 0.61) with 2.82 < Q2 < 4.8 GeV2 using deep inelastic electron scattering on a polarized 3He target.

The spin structure of the neutron is probed by A1n. At large x, the structure of the neutron should be dominated by valence quark behavior. A simple SU(6) quark-model wavefunction predicts that A1n = 0 for all x. However, broken SU(6) and perturbative QCD models both predict that A1n should approach unity as x -> 1. All previous data above about x = 0.4 have large uncertainties, and cannot distinguish between the predictions. This experiment, which took data in the summer of 2001, will produce results with dramatically smaller error bars, and should allow for the first time the ability to distinguish between these model predictions.
GDH sum rule at small angles    JLab Experiment 97-110

There are clean theoretical predictions, in the form of sum rules, for the spin structure of the nucleon both at small Q2, where hadronic degrees of freedom dominate, and in the Q2 -> infinity scaling limit, where quark degrees of freedom most accurately describe the underlying structure. At Q2=0, the Gerasimov-Drell-Hearn (GDH) sum rule relates the integrated cross section difference for real photon absorption by a nucleon to the anomalous magnetic moment of the nucleon.

Recently, it has been recognized that an extended GDH sum rule can also be written, which allows investigation of the region between Q2=0 and Q2=1.0 GeV2, where one makes a transition from a hadronic, to a quark-model description of the nucleon. By studying this transition region, we have an opportunity to learn how QCD forces the individual quarks to begin to behave like a coherent nucleon.

Experiment E97-110 will measure the asymmetries (and cross section differences) for polarized electron scattering from polarized 3He using the new Hall A septum magnets. These will allow data to be obtained at scattering angles as small as 6 degrees. This measurement will determine the GDH sum rule from Q2=0.5 down to 0.02 GeV2, nearly an order of magnitude lower than in previous experiments. The experiment will take place in the fall of 2002, and will form the basis for Vince Sulkovsky's PhD thesis.

Helium3 Target Lab

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Physics Department
Graduate Studies in Physics at William and Mary
Jefferson Lab

armd@physics.wm.edu
last updated: June 3 2002