Precision measurement of the neutron spin asymmetries and spin-dependent structure functions in the valence quark region

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 $4.8\phantom{\rule{0.3em}{0ex}}{(\mathrm{GeV}∕c)}^{2}$, respectively. These measurements were performed using a $5.7\phantom{\rule{0.3em}{0ex}}\mathrm{GeV}$ longitudinally polarized electron beam and a polarized $^{3}\mathrm{He}$ 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 2. 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 $\ensuremath{\Delta}u∕u$ agree well with predictions from various models, results for $\ensuremath{\Delta}d∕d$ disagree with the leading-order PQCD prediction when hadron helicity conservation is imposed.