Measuring the Neutron and 3 He Spin Structure at Low Q 2 Vincent Sulkosky College of William and Mary, Williamsburg VA Experimental Overview The goal of Jefferson Lab experiment E is to study neutron and 3 He spin structure by performing a precise measurement of the generalized Gerasimov-Drell-Hearn (GDH) integral at Q 2 between 0.02 and 0.3 GeV 2. The Experiment was run in summer 2003 in Hall A. Experimental Setup Polarized electron beam, average P beam ~ 75% Hall A polarized 3 He 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 3 He target Polarized 3 He Target Optical pumping of Rb atoms Spin exchange between Rb atoms and 3 He nuclei Target cells: 40 cm, ~ 10 atm Highest polarized luminosity in the world: up to a few x s -1 P targ = 40% Two independent polarimetries: NMR and EPR Why 3 He as an effective n target? 3 He = 3 He n Effective polarized neutron target 3 He 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 angular distribution in the target region. For a carbon foil target located at the origin of Hall A. Particle trajectories passing through the spectrometer from the target region to the spectrometer focal plane. Corresponding coordinates at the focal plane. x sieve is related to fp, and tg, to y fp. Target Septum 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 (Q 2 = 0) Sum Rule Static Properties 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 Q 2. Generalized GDH (Q 2 > 0) Replace photoproduction cross sections with electroproduction (virtual photons). Previous JLab experiment E94-010: Measured generalized GDH on neutron with Q 2 between 0.1 to 0.9 GeV 2. Studied transition between strong interactions partonic to hadronic descriptions. Results did not agree well with Chiral perturbation theory above 0.1 GeV 2. Present work, JLab experiment E97-110: Check Chiral perturbation theory ( PT) in a region where it is valid. Extrapolate to the real point (Q 2 = 0). Analysis Overview E expected accuracy for the neutron generalized GDH integral. The red triangles show the E results. The blue circles show the Q 2 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