Measurements of d_{2}^{n} and A_{1}^{n} : Probing the neutron spin structure
Abstract
We report on the results of the E06014 experiment performed at Jefferson Lab in Hall A, where a precision measurement of the twist3 matrix element d_{2} of the neutron (d_{2}^{n}) was conducted. The quantity d_{2}^{n} represents the average color Lorentz force a struck quark experiences in a deep inelastic electron scattering event off a neutron due to its interaction with the hadronizing remnants. This color force was determined from a linear combination of the third moments of the ^{3}He spin structure functions, g_{1} and g_{2}, after nuclear corrections had been applied to these moments. The structure functions were obtained from a measurement of the unpolarized cross section and of doublespin asymmetries in the scattering of a longitudinally polarized electron beam from a transversely and a longitudinally polarized ^{3}He target. The measurement kinematics included two average Q^{2} bins of 3.2 GeV^{2} and 4.3 GeV^{2} , and Bjorkenx 0.25 ≤x ≤0.90 covering the deep inelastic and resonance regions. We have found that d_{2}^{n} is small and negative for ⟨Q^{2}⟩ =3.2 GeV^{2} , and even smaller for ⟨Q^{2}⟩ =4.3 GeV^{2} , consistent with the results of a lattice QCD calculation. The twist4 matrix element f_{2}^{n} was extracted by combining our measured d_{2}^{n} with the world data on the first moment in x of g_{1}^{n}, Γ_{1}^{n}. We found f_{2}^{n} to be roughly an order of magnitude larger than d_{2}^{n}. Utilizing the extracted d_{2}^{n} and f_{2}^{n} data, we separated the Lorentz color force into its electric and magnetic components, F_{E}^{y ,n} and F_{B}^{y ,n}, and found them to be equal and opposite in magnitude, in agreement with the predictions from an instanton model but not with those from QCD sum rules. Furthermore, using the measured doublespin asymmetries, we have extracted the virtual photonnucleon asymmetry on the neutron A_{1}^{n}, the structure function ratio g_{1}^{n}/F_{1}^{n}, and the quark ratios (∆ u +∆ u ̄)/(u +u ̄) and (∆ d +∆ d ̄)/(d +d ̄). These results were found to be consistent with deepinelastic scattering world data and with the prediction of the constituent quark model but at odds with the perturbative quantum chromodynamics predictions at large x .
 Publication:

Physical Review D
 Pub Date:
 September 2016
 DOI:
 10.1103/PhysRevD.94.052003
 arXiv:
 arXiv:1603.03612
 Bibcode:
 2016PhRvD..94e2003F
 Keywords:

 Nuclear Experiment;
 Nuclear Theory
 EPrint:
 55 Pages, 45 figures, 48 tables. 15 supplemental files. Grammatical and typo fixes. Revised some model descriptions and updated analyses for f_2^n and color forces