Forward Meson Production and Spin Asymmetries at RHIC

现在一个公认的事实是，核子、质子和中子是由更基本的粒子——夸克组成的。该基金支持的主要物理项目是研究核子和原子核的夸克结构。作为STAR合作项目的一部分，研究将在布鲁克海文国家实验室进行。这次合作的重点是高能极化质子从原子核的散射，目的是了解质子的结构。拟议中的研究将使用前向电磁量热计，称为FMS，它是STAR探测器系统的一部分，用于测量以小散射角产生的中性粒子。人们发现，这种粒子产生的测量速率在很大程度上依赖于质子束的极化方向，这一效应使基于强作用力基本理论量子色动力学（QCD）的理论理解的尝试变得混乱。利用FMS对这种粒子的进一步研究将指导未来的理论发展，并帮助我们更多地了解核子的夸克和胶子结构。该提案支持宾夕法尼亚州立大学小组在STAR实验中操作FMS探测器进行极化质子RHIC运行，并分析这些运行的数据。STAR FMS是一种电磁量热计，用于检测前方伪快区产生的碰撞碎片。在这个区域观察到的粒子与能量最高和极化程度最高的夸克的相互作用有关。对于横向极化的质子，横截面对极化符号的很大依赖是一个长期存在的谜。虽然QCD拉格朗日量是已知的，但我们用它来预测可观测值的能力仍然有限。在电磁力中，原子的自旋成分的作用已经被很好地理解，与此相反，自旋在部分（束缚态质子的成分）中的作用仍然接近我们的知识前沿。量子cd力散射最有用的模型是微扰量子cd，或pQCD，它适用于质子组分之间的碰撞交换大动量的情况。本研究的目标是通过实验确定pQCD应用的地方以及如何修改模型以扩展适用性。

It is now a well-established fact that nucleons, protons and neutrons, are made up of more elementary particles called quarks. The main physics program supported by this grant is directed at the study of the quark structure of nucleons and nuclei. Research will be carried out at the Brookhaven National Laboratory as part of a collaboration called STAR. This collaboration is focused on the scattering of high energy polarized protons from nuclei with the goal of understanding the structure of the proton. The proposed research will use the forward electromagnetic calorimeter, called FMS, which is part of the STAR detector system to measure neutral particles that are produced at small scattering angles. The measured rates of such particle production have been found to depend strongly on the direction of polarization of the proton beam, an effect that has confounded attempts at a theoretical understanding based on the fundamental theory of the strong force, Quantum Chromodynamics (QCD). Further studies of this particles detected using the FMS will guide future theoretical develpments and help us to understand more about the quark and gluon structure of nucleons.This proposal supports the Penn State group to operate the FMS detector in the STAR experiment for polarized proton RHIC runs and to analyze data from those runs. The STAR FMS is an electromagnetic calorimeter that detects collision fragments produced in the forward pseudo-rapidity region. Particles observed in this region are associated with the interactions of the most energetic and most polarized quarks. For transversely polarized protons, the large dependence of the cross section on the sign of polarization is a long-standing mystery. While the QCD Lagrangian is known, our ability to use it to predict observables is still limited. In contrast to the electromagnetic force, where the role of spin constituents of an atom is fairly well understood, the role of spin among the partons (constituents of a bound state proton) remains near our knowledge frontier. The most useful model for scattering by the QCD force is perturbative QCD, or pQCD, which is applicable when the collisions between protons constituents exchange large momentum. The goal of this research is to determine experimentally where pQCD applies and how to modify models to extend applicability.