"Figure 7 Theory 1" of "Neutral pion cross section and spin asymmetries at intermediate pseudorapidity in polarized proton collisions at $\sqrt{s} = 200$ GeV"

The differential cross section and spin asymmetries for neutral pions produced within the intermediate pseudorapidity range 0.8 < $\eta$ < 2.0 in polarized proton-proton collisions at $sqrt{s}$ = 200 GeV are presented. Neutral pions were detected using the endcap electromagnetic calorimeter in the STAR detector at RHIC. The cross section was measured over a transverse momentum range of 5 < $p_{T}$ < 16 GeV/c and is found to be within the scale uncertainty of a next-to-leading order perturbative QCD calculation. The longitudinal double-spin asymmetry, $A_{LL}$, is measured in the same pseudorapidity range. This quantity is sensitive to the gluonic contribution to the proton spin, $\Delta$g(x), at low Bjorken-x (down to x $\approx$ 0.01), where it is less constrained by measurements at central pseudorapidity. The measured $A_{LL}$ is consistent with model predictions. The parity-violating asymmetry, $A_{L}$, is also measured and found to be consistent with zero. The transverse single-spin asymmetry, $A_{N}$, is measured within a previously unexplored kinematic range in Feynman-x and $p_{T}$. Such measurements may aid our understanding of the on-set and kinematic dependence of the large asymmetries observed at more forward pseudorapidity ($\eta \approx 3$) and their underlying mechanisms. The $A_{N}$ results presented are consistent with a twist-3 model prediction of a small asymmetry within the present kinematic range.

本文呈现了质心系碰撞能量√s=200 GeV的极化质子-质子碰撞中，产生于赝快度0.8 < η < 2.0中间区间的中性π介子（neutral pions）的微分截面（differential cross section）与自旋不对称度（spin asymmetries）。中性π介子通过相对论重离子对撞机（Relativistic Heavy Ion Collider, RHIC）上STAR探测器（STAR detector）的端盖电磁量能器（endcap electromagnetic calorimeter）完成探测。我们在5 < p_T < 16 GeV/c的横向动量（transverse momentum）区间内测得该微分截面，其结果处于次领头阶微扰量子色动力学（next-to-leading order perturbative QCD, pQCD）计算的标度不确定度范围内。我们在相同的赝快度区间内测量了纵向双自旋不对称度（longitudinal double-spin asymmetry）A_LL，该物理量在低布约肯x（Bjorken-x，低至x≈0.01）区域对质子自旋的胶子贡献Δg(x)敏感，而中心赝快度（central pseudorapidity）区间的测量对该区域的约束能力较弱。测得的A_LL与现有模型预言结果一致。我们同时测量了宇称破缺不对称度（parity-violating asymmetry）A_L，结果与零值相符。我们在费曼x（Feynman-x）与p_T的此前未被探索过的运动学区间内，测量了横向单自旋不对称度（transverse single-spin asymmetry）A_N。此类测量有助于我们理解在更向前的赝快度区域（η≈3）观测到的大自旋不对称度的起源、运动学依赖关系及其背后的物理机制。本文给出的A_N结果与当前运动学区间内扭曲3模型（twist-3 model）预言的小不对称度相符。