Precision Measurement of the Neutron Spin Asymmetries and Spin-dependent Structure Functions in the Valence Quark Region
X. Zheng
TL;DR
The paper delivers high-precision neutron spin observables in the valence region by scattering polarized electrons off polarized ^3He and extracting A1^n, A2^n, g1^n, and g2^n at x up to 0.60. Using rigorous nuclear corrections and radiative analyses, it finds A1^n crossing zero near x≈0.47 and becoming positive at larger x, with A2^n and g2^n measured to high precision and the higher-twist matrix element d2^n showing a positive signal. The results, combined with world proton data, yield polarized quark distributions: Δu/u agrees with several models, while Δd/d challenges LO pQCD with hadron helicity conservation, suggesting significant quark orbital angular momentum effects in the valence region. Overall, the work provides critical inputs for QCD-based PDFs and tests of nucleon spin structure in a regime where valence quarks dominate. It also demonstrates the efficacy of a ^3He target as an effective neutron probe and reinforces the role of higher-twist dynamics in spin-dependent observables.
Abstract
We report on measurements of the neutron spin asymmetries $A_{1,2}^n$ and polarized structure functions $g_{1,2}^n$ at three kinematics in the deep inelastic region, with $x=0.33$, 0.47 and 0.60 and $Q^2=2.7$, 3.5 and 4.8 (GeV/c)$^2$, respectively. These measurements were performed using a 5.7 GeV longitudinally-polarized electron beam and a polarized $^3$He target. The results for $A_1^n$ and $g_1^n$ at $x=0.33$ are consistent with previous world data and, at the two higher $x$ points, have improved the precision of the world data by about an order of magnitude. The new $A_1^n$ data show a zero crossing around $x=0.47$ and the value at $x=0.60$ is significantly positive. These results agree with a next-to-leading order QCD analysis of previous world data. The trend of data at high $x$ agrees with constituent quark model predictions but disagrees with that from leading-order perturbative QCD (pQCD) assuming hadron helicity conservation. Results for $A_2^n$ and $g_2^n$ have a precision comparable to the best world data in this kinematic region. Combined with previous world data, the moment $d_2^n$ was evaluated and the new result has improved the precision of this quantity by about a factor of two. When combined with the world proton data, polarized quark distribution functions were extracted from the new $g_1^n/F_1^n$ values based on the quark parton model. While results for $Δu/u$ agree well with predictions from various models, results for $Δd/d$ disagree with the leading-order pQCD prediction when hadron helicity conservation is imposed.