Measuring the Neutron and 3He Spin Structure at Low Q2

Vincent Sulkoskyfor the JLab Hall A Collaboration

College of William and Mary, Williamsburg VA 23187

Experimental Overview

The goal of Jefferson Lab experiment E97-110 is to study neutron and 3Hespin structure by performing a precise measurement of the generalizedGerasimov-Drell-Hearn (GDH) integral at Q2 between 0.02 and 0.3 GeV2. The Experiment was run in summer 2003 in Hall A.

Experimental Setup

• Polarized electron beam, average Pbeam ~ 75%

• Current ~ 1-12 A

• Hall A polarized 3He target (as effective neutron target)

• Scattered electrons detected by Hall A High Resolution Spectrometer coupled with a septum magnet (inclusive reaction).

• Septum magnet: horizontal bending dipole magnet that enabled detection of electrons at 6 and 9 degrees.

Floor layout in Hall A.The septum magnet.

Polarized 3He Target

• Optical pumping of Rb atoms

• Spin exchange between Rb atoms and 3He nuclei

• Target cells: 40 cm, ~ 10 atm

• Highest polarized luminosity in the world: up to 1036 cm-2 s-1

Effective polarizedneutron target

3He as an effective n target:

3He = 3He n

3He standard target cell.

Polarized RBand 3He

Polarized 3He only

220 oC

50 oC

Special cell designed for forward angledetection.

Electron Beam

Scattered Electrons

The neutron GDH Experiments at JLab Hall A

GDH Sum Rule (Q2 = 0)

Sum Rule Static Properties

2

32

10

d

2

2

22 M

measured theory well known

• Can be used to check theory or measure static properties.

• and: cross sections for photoproduction with two different photon polarizations.

• Can be generalized for nonzero Q2.

Generalized GDH (Q2 > 0)

• Replace photoproduction cross sections with electroproduction (virtual photons).

• Previous JLab experiment E94-010:

Measured generalized GDH on neutron with Q2 between 0.1 to 0.9 GeV2.

Studied transition between strong interaction’s partonic to hadronic descriptions. Results did not agree well with

Chiral perturbation theory above 0.1 GeV2.

• Present work, JLab experiment E97-110:

Benchmark test of Chiral perturbation theory (PT) in a region where it should be valid.

Extrapolate to the real point (Q2 = 0).

Target Apparatus and Performance

Polarized target setup.

• Longitudinally and transversely polarized target.

• Ptarg = 38.5% (from on-line analysis).

• Two independent polarimetries: NMR and EPR.

Target performance during experiment E97-110.

Analysis Overview and Progress

Expected Results

Beam line: beam polarization,current calibration, energy measurements, etc.

Elastic analysis and background

Detector calibrations and efficiencies: VDC, gas Cherenkov,and shower calorimeters

Spectrometer optics and acceptance

Target polarimetry

Asymmetries and cross sections

Radiative corrections

E97-110 expected accuracy for the neutron generalized GDH integral. The red circles show the E94-010 results. The blue circles show the Q2 range, and the blue band shows the expected systematic uncertainty. The vertical axis has been normalized to the neutron value at the real photon point (233.2 b).

Carbon foil position reconstruction along thebeam line at four different momentum settings within 4% of the elastic peak. The vertical lines represent the nominal foil positions.

= + 2% = + 0% = - 2% = - 4%

Energy deposited in total shower calorimeterafter calibration.

Plot of 3He elastic asymmetry showing the fourdifferent target and beam configurations. Simulation: 1.39%. Preliminary data analysis: (1.403 0.044)% (stat. only).