A Measurement Of The Proton’s Spin Structure Function G₂ At Low Q²

JLab E08-027, a measurement of g₂^p and the longitudinal-transverse （LT） spin po-larizability, successfully collected data from March to May,2012. Nucleon spin struc-ture study has been an active research area, which has attracted a very large effort from both experimentalists and theorists. The spin structure study for the last 2 decades has provided us with many exciting and often surprising results. Recently, new precision results in the low-to-intermediate momentum transfer Q² region from JLab have pro-vided extensive information on the nucleon structure in the confinement region and the transition region between asymptotic free to confinement. In particular, the exten-sive comparisons of experimental results with Chiral Perturbation Theory （the effective theory of QCD at low energy） calculations show general good agreements, but strong disagreement in the case of the neutron LT spin polarizability. This experiment com-pleted the measurements of g₂^p and the LT spin polarizability on the proton in the low-to-intermediate Q² region.The experiment used a polarized proton （NH₃） target for the first time in Hall A. Scattered electrons were detected by a pair of Hall A high resolution spectrometer （HRS） with a pair of septum magnets. To avoid too much depolarization of the target, beam current was limited to 50-100 nA during the experiment. Since the existing beam current monitors （BCMs）, beam position monitors （BPMs） and calibration methods did not work at such a low current range, new BPM and BCM receivers were designed and used for current condition. A pair of super-harps and a tungsten calorimeter were installed to calibrate the BPMs and BCMs. To compensate for the effect of the 2.5/5T transverse magnet field, two chicane dipole magnets were installed. A pair of slow rasters were installed for the first time in Hall A, combining with a pair of fast raster. The standard Hall A DAQ system and the improved high resolution DAQ system were used to record the detector information and the helicity dependent beam information, respectively.In order to achieve the required accuracy of the beam position and angle and recon-struct them event by event at the target location, the data of the BPMs and harps were carefully analyzed. The final uncertainty of BCM after the calibration using the tung-sten calorimeter is below 1%, which is important for the asymmetry extraction. Before the acceptance and the dilution factor are available, the models from fitting to the world data were used instead to extract the unpolarized cross section and the dilution factor. The longitudinal and transverse physics asymmetries were extracted and compared with the model, with the study of the radiative correction. The preliminary g₁ and g₂ results were then extracted using the measured asymmetry and the model.