Measuring the Neutron and 3He Spin Structure at Low Q2

Vincent Sulkosky

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%

• 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.

The septum magnet The Hall A Pivot with Polarized 3He target

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 a few x 1036 s-1

• Ptarg = 40%

• Two independent polarimetries: NMR and EPR

Why 3He as an effective n target?

3He = 3He nEffective polarizedneutron target

3He target cell

My Contribution

• Spectrometer optics calibration and acceptance study

Accurate knowledge of the spectrometer magnetic optics is required to obtain precise cross section measurements

Septum magnet changes optical properties of spectrometer

and requires careful study and calibration of the optics.

Out-of-plane and in-plane angulardistribution in the target region. For a carbon foil target located atthe origin of Hall A.

Particle trajectories passing through the spectrometer from the target region to the spectrometer focal plane.

Corresponding coordinates atthe focal plane. xsieve is relatedto fp, and tg, to yfp.

TargetSeptum

Q1

Q2

Dipole

Q3

Focal Plane

• Optics calibration: optimize the matrix coefficients that link the target coordinates to the focal plane coordinates

• Calibration completed for both scattering angles (6 and 9 degrees), total of five beam energies.

• Target reconstruction accuracy comparable to spectrometer without septum.

• Responsible for target NMR system prior to and during experiment

• Septum field ~ 400 times larger than target field, ~ 1m from target center

• Field gradients > 30 mGauss/cm: lower target maximum polarization and cause significant polarization loss

• Mapped target field prior to the experiment with septum magnet

NMR System

• Field clamps and compensation coils were used to reduce gradients at the target.

• Reduced gradients did not cause significant polarization loss.

Field gradient along the target cell in the direction of the holding field with respect to different septum currents.

Toward septum

Away from septum

Target center

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.

• 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:

Check Chiral perturbation theory (PT) in a region where it is valid.

Extrapolate to the real point (Q2 = 0).

Analysis Overview

E97-110 expected accuracy for the neutron generalized GDH integral. The red triangles show the E94-010 results. The blue circles show the Q2 range, and the band shows the expected

systematic uncertainty.

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

Elastic analysis and background Studies

Detector Calibrations and efficiencies: VDC, gas Cherenkov,and shower calorimeters.

Spectrometer optics and acceptance

Target polarimetry

Asymmetries and cross sections