"Figure 4 left panel" 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.

本文呈现了质心系能量sqrt{s}=200 GeV的极化质子-质子碰撞中，赝快度区间0.8 < eta < 2.0的中间赝快度范围内产生的中性π介子（neutral pion）的微分截面与自旋不对称度。中性π介子采用相对论重离子对撞机（Relativistic Heavy Ion Collider, RHIC）上STAR探测器的端盖电磁量能器进行探测。本次测量的微分截面覆盖横向动量5 < p_T < 16 GeV/c的区间，其结果处于次领头阶微扰量子色动力学（next-to-leading order perturbative QCD）计算的标度不确定度范围内。纵向双自旋不对称度（longitudinal double-spin asymmetry）A_{LL}在相同赝快度区间内被测量，该物理量对低比约肯x（Bjorken-x，低至xapprox0.01）处质子自旋的胶子贡献Delta g(x)敏感，而该能区在中心赝快度区的现有测量数据约束相对不足。本次测得的A_{LL}与模型预言结果一致。宇称破缺不对称度（parity-violating asymmetry）A_L同样被测量，其结果与零值相符。横向单自旋不对称度（transverse single-spin asymmetry）A_N在费曼x（Feynman-x）与横向动量的全新运动学区间内被测量。此类测量有助于理解更大前向赝快度区（etaapprox3）观测到的大不对称现象的起源与运动学依赖关系，以及其背后的物理机制。本次给出的A_N结果与扭曲3（twist-3）模型的预言一致，该模型预言在当前运动学区间内不对称度较小。