Measuring the Spin Structure of 3He and the Neutron at Low Q2

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Measuring the Spin Structure of 3He and the
Neutron at Low Q2

• Timothy Holmstrom
• College of William and Mary
• For the Jefferson Lab Hall A Collaboration
• DNP 2005
• Maui, Hawaii, September 22, 2005

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Polarized Inclusive Electron Scattering

• Probe the response of the entire nucleus
  structure.
• Provides a way to test QCD.
• Measures the scattering of all interactions.
• Dominated by one-photon exchange.

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Gerasimov-Drell-Hearn Sum Rule

• Relates the difference between polarized
  photo-production cross sections of real photons
  (Q2 0) aligned and anti-aligned with the
  targets spin to the anomalous magnetic moment.
• Has been used in analysis and theoretical
  predictions.
• For spin ½ targets it is

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The Generalized GDH Integral

• Extends the GDH Sum Rule to the finite Q2 region.
• Allows the comparison of experimentally measured
  quantities at finite Q2s with theoretical
  predictions.
• Replaces the photoproduction cross-sections with
  the electroproduction ones.
• Related to the forward Compton scattering
  amplitude (S1(2)), which can calculated by theory
  on full nucleon spectrum.

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JLab Experiment E94010

• Extended GDH integral on the Neutron at
  intermediate Q2 (0.1-0.9 GeV2).
  (PRL 89, 242301-1 (2002))
• Used the Hall A HRSs.
• Polarized 3He target.
• An extension from the known region of QCD to the
  unknown.
• The lowest Q2 point compares with chiral
  perturbation theory calculations.

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Small Angle GDH (E97110)

• Precise measurement of the Q2 dependence of the
  generalized GDH integral and the moments of the
  3He and neutron spin structure functions between
  0.02 to 0.3 GeV2.
• Measurement of asymmetries and cross sections
  will allow us to
• Determine the slope of the extended GDH integral
  as Q2 approaches 0.
• Test the applicability of Chiral Perturbation at
  low Q2.
• Extrapolate to the real photon point for 3He and
  the Neutron
• Constrains the resonance structure of the
  nucleon.
• The results will also compliment the e94010 Q2
  results from 0.1 to 0.3 GeV2 with higher
  precision.
• The experiment was run successfully in the spring
  and summer of 2003 using the right Hall A High
  Resolution Spectrometer.
• A new septum magnet was used to measure inclusive
  electron scattering at 6 and 9.

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Jefferson Lab Hall A
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Polarized 3He Target

• The 3He is polarized through spin exchange with
  optically pumped Rb atoms.
• Two methods of polarimetry NMR and EPR.
• Target polarization in beam 38.5.
• Longitudinal and transverse polarization
  configurations were used.

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Kinematics Coverage

• The integral will be calculated between Q2 of
  0.02GeV2 and 0.3GeV2.
• The high level overlap will allow us the minimal
  amount of interpellation.

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Expected Results
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Elastic Asymmetry

• Elastic Scattering at 9 with a beam energy of
  1.142 GeV.
• Four different target and beam configurations
  agree with each other.
• Preliminary analysis agrees with Monte Carlo
  predictions.

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Septum Optics and Acceptance

• Calculation of the magnetic database including
  the septum is now complete. Results look good at
  both 6? and 9?.
• Extensive studies are currently being preformed
  to understand our acceptance with the septum
  magnet.

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Conclusions

• The low Q2 GDH experiment was successfully run at
  Jefferson Lab in the summer of 2003.
• The experiment will provide new low Q2
  measurements of the extended GDH integral, which
  will
• check the limits of applicability of Chiral
  Perturbation Theory
• determine where or if the slope turns over, which
  is important for nucleon structure resonances
• allow an extrapolation to the real photon point.
• The data complements the E94010 data above Q2 of
  0.1 with better precision.
• Elastic asymmetry analysis looks good
• The septum optics is complete, and analysis of
  the acceptance is going well.
• We hope to have preliminary cross sections by the
  end of the year.

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Using 3He as a Neutron Target

• 3He nucleons come in three possible states S, D,
  S'.
• The S State dominates leading to an effective
  neutron polarization of 86. (J. Friar et al.).