A measurement of the proton’s spin structure function g2 at low Q2

Abstract

JLab E08-027, a measurement of g2p and the longitudinal-transverse (LT) spin polarizability, successfully collected data from March to May, 2012. Nucleon spin structure 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 Q 2 region from JLab have provided extensive information on the nucleon structure in the confinement region and the transition region between asymptotic free to confinement. In particular, the extensive 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 completed the measurements of g p 2 and the LT spin polarizability on the proton in the lowto-intermediate Q 2 region.The experiment used a polarized proton (N H 3 ) 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 reconstruct 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 tungsten 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 g 1 and g 2 results were then extracted using the measured asymmetry and the model.

The g$p\atop{2}$ Experiment: A Measurement of the Proton's Spin Structure Functions

“…For example, the spin structure of the proton has a noticeable effect on the energy levels in atomic hydrogen. This is remarkable considering the separation in energy and length scales: the energy of electron orbitals in hydrogen is on the order of eV, but the proton mass is almost a GeV, and the electron is, on average, 5×10 −11 m away from the proton of approximate radius The results of this thesis represent significant progress from the previous E08-027 theses [144,178,192,193,194], including a full characterization of the radiation corrections procedure on the unique angular dependence present in the data. The statistics presented are the full analyzing power of the 5 T kinematic settings.…”