"Figure 2 right 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.

本数据集展示了质心系能量√s=200 GeV的极化质子-质子碰撞中，赝快度区间0.8 < η < 2.0的中间快度区域内产生的中性π介子的微分截面（differential cross section）与自旋不对称性（spin asymmetries）。中性π介子通过相对论重离子对撞机（RHIC）上的STAR探测器的端盖电磁量能器进行探测。本实验在横向动量5 < p_T < 16 GeV/c的区间内测量了微分截面，结果处于次领头阶（next-to-leading order）微扰量子色动力学（perturbative QCD）计算的标度不确定度范围内。同一赝快度区间内还测量了纵向双自旋不对称性A_{LL}，该物理量对低比约肯x（Bjorken-x，低至x≈0.01）处质子自旋的胶子贡献Δg(x)敏感，而中心赝快度区域的测量结果对该物理量的约束较弱。本次测量得到的A_{LL}与模型预言相符。宇称破缺不对称性A_L也被测量，结果与零一致。横向单自旋不对称性A_N在此前未被探索的费曼x（Feynman-x）与p_T运动学区间内完成测量，这类测量有助于理解在更靠前赝快度区域（η≈3）观测到的大自旋不对称性的起源、运动学依赖关系及其内在机制。本次给出的A_N结果与twist-3模型（twist-3 model）在当前运动学区间内预言的小不对称性相符。