Measuring the Neutron and 3 He Spin Structure at Low Q 2 Vincent Sulkosky for the JLab Hall A Collaboration 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% Current ~ 1-12 A 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. Floor layout in Hall A. The septum magnet. 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 cm -2 s -1 Effective polarized neutron target 3 He as an effective n target: 3 He = 3 He n 3 He standard target cell. Polarized RB and 3 He Polarized 3 He only 220 o C 50 o C Special cell designed for forward angle detection. Electron Beam Scattered Electrons 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. Can be 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: Benchmark test of Chiral perturbation theory ( PT) in a region where it should be valid. Extrapolate to the real point (Q 2 = 0). Target Apparatus and Performance Polarized target setup. Longitudinally and transversely polarized target. P targ = 38.5% (from on-line analysis). Two independent polarimetries: NMR and EPR. Target performance during experiment E 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 E expected accuracy for the neutron generalized GDH integral. The red circles show the E results. The blue circles show the Q 2 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 the beam 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 calorimeter after calibration. Plot of 3He elastic asymmetry showing the four different target and beam configurations. Simulation: 1.39%. Preliminary data analysis: ( )% (stat. only).