Precision measurement of neutron spin asymmetry An1 at large xBj using CEBAF at 5.7 GeV

Abstract

Nucleon spin structure has remained one of the key issues of hadronic physics since the 1980's. Among the unsolved questions, the neutron spin structure in the valence quark region (the large Bjorken variable XBj region) is of particular interest. The few data that existed in the large XBj region are very imprecise. Also, theoretical predictions based on Quantum Chromo-Dynamics (QCD) are difficult to carry out accurately in this non-perturbative region and having good experimental data is very important to test the models. Taking advantage of the polarized continuous-wave (CW) electron beam at Jefferson Laboratory (JLAB) and a high-density polarized 3He target in Hall A, we have measured the neutron spin asymmetry An1 and spin structure function gn1 to a good precision at three kinematics x = 0.33, 0.47 and 0.60, with Q2 = 2.7, 3.6 and 4.9 (GeV/c)2, respectively. The data at the two higher XBj points have improved the statistical accuracy of the world data by one order of magnitude. They show a zero crossing point around XBj = 0.47 and the datum at XBj = 0.60 is significantly positive. To determine An and g, cross sections and asymmetries of inclusive 3He(e,e') scattering have been measured in the deep-inelastic region. The transverse spin asymmetry An and structure function g were also extracted from data, though with less statistical precision. The asymmetries of pion photoproduction A[pi]⁻ were obtained as a byproduct.

(cont.) Combined with world fit of proton gP1/Ff1 data and the quark distribution ratio d/u, the polarized quark distributions ... have been obtained from gn1/Fn1 data. Results of ... at all three XBj points are negative, in disagreement with the predictions from pQCD based hadron helicity conservation. This dissertation will first give an introduction to the theories and formalism of polarized deep inelastic scattering and a review of the theories of An1. Next the experiment E99-117 at JLAB Hall A will be described, followed by the data analysis which yields results for ... The data presented greatly improve the current world fit of neutron polarized structure functions and provide valuable insight in the understanding of the neutron spin structure.