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项目名称：

STAR前向粒子关联和超子极化研究QGP和冷核结构性质

摘要：

相对论重离子碰撞的重要前沿方向是研究量子色动力学（QCD）中夸克胶子等离子体(QGP)的特性和冷核中夸克胶子的分布和涨落。在 RHIC第一期能量扫描的数据中，STAR在2017年发现了由中国学者领头提出的新现象：超子整体极化 (GHP)。GHP可以由QGP流体涡旋产生且有很强的快度分布，是探测QGP粘滞系数和内部自由度的新手段。经过RHIC和LHC十几年的努力，对QGP流体的研究已经从单一测量发展到精准的谐波分析及其快度退关联。喷注淬火和流的大快度关联也成为研究小系统碰撞和QGP性质的必要手段。 前向大快度的粒子关联和Drell-Yan双轻子在极化质子碰撞中的测量是研究夸克胶子在冷核中的分布的重要手段。STAR作为 RHIC现存唯一的大型探测器，在今后几年内将独自承担寻找QCD相变点和极化强子碰撞对核子自旋结构研究的重要任务。前向大快度的径迹探测和量能器成了必不可少的升级。本项目计划对上述前向粒子关联和超子极化物理实验和理论研究有机地结合，并将承担STAR前向硅条探测器的预研和部分建造任务。

英文摘要：

The frontiers of Relativistic heavy-ion physics are to study the properties of Quark-Gluon Plasma (QGP), and the parton distributions and fluctuations in nucleons and nuclei. Over the last decade, RHIC and the LHC scientific programs have progressed from qualitative assessment of QGP hydrodynamic feature by elliptic flow measurement toward quantitative measurements of the QGP transport coefficients and initial state gluon fluctuations through multiple harmonic flow analyses and their rapidity decorrelations. The coupling between quenched jet and medium flow over large rapidity range has been extensively studied in small-system collisions and has become an important tool in studying the jet quenching mechanism in QGP. In 2017, STAR Collaboration has published in NATURE cover page the discovery of Global Hyperon Polarization (GHP) in the most vortical fluid known. GHP was stipulated to exist in swirling QGP by Chinese theorists in 2005, and is predicted to grow with rapidity due to the increase of QGP hydrodynamic viscosity. Even with the multiple major upgrades in recent years, STAR detector is only capable of tracking and particle identification within pseudorapdity of |h|<1.5. This proposal seeks to research on particle and flow correlations over large rapidity range and to study the rapidity dependence of the newly discovered GHP at RHIC. This research program requires upgrades to extend the STAR detector at the forward rapidity to 2.5<h<4. We plan to collaborate with two other major institutions in the USA within the STAR Collaboration to carry out R&D on Silicon Strip technology necessary for the hyperon reconstruction at forward rapidity and to support a fraction of the construction of this detector in ready for data-taking after 2020. The proposed detector instrumentation is also necessary for further study of the spin structure of nucleon using di-hadron correlation and Drell-Yan production at forward rapidity in the unique polarization p+p collisions at RHIC. These R&D and physics measurements are important studies in realistic environment at RHIC toward realization of the forward instrumentation at a future electron-ion collider. 

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立项依据：

1      项目的研究意义及科学依据

强相互作用力是自然界的四大作用力之一。强相互作用的基本作用过程由色动力学（Quantum Chromodynamics, QCD）描述，它具有渐近自由和色禁闭两大特性，作用力的传播子是胶子。 格点量子色动力学是数据模拟色动力学中凝聚态行为的理论框架。该理论描绘了一个强相互作用物质的相图：在极端高温与高能量密度条件下核物质可以发生从强子相到夸克相的转变，形成色禁闭解除的物质。这一新的物质形态被称为夸克胶子等离子体(Quark-Gluon Plasma, 简称 QGP)。人类对四大作用力中的电磁相互作用力和万有引力的了解和应用广泛。随着科技飞速发展，人们对超强电磁场和超强引力场的研究方兴未艾。相对论重离子碰撞也为产生和研究超强色动力学的高温高密胶子场提供了一个理想的场所。这为探索真空对称结构和激化，QCD核物质相图、非微扰QCD 和夸克禁闭机制等基本科学问题提供重要的实验手段。

作为作用力传播子的胶子可以在夸克胶子和胶子胶子之间相互作用。宇宙中99%以上的可见物质是由它们相互作用的动力学产生的。当前核物理的一个热门研究前沿是核子中胶子和海夸克的分布和关联：1. 胶子和海夸克的自旋，时空和动量在核子中的分布情况；轨道角动量对核子质量构造的贡献成分有多大；2. 从质子到重核子，胶子和海夸克分布和关联的演化有没有从高密度的量变到胶子饱和(gluon saturation)的质变过程？存不存在高密度的胶子的新形态：色玻璃体(color glass condensate)？3. 高能量的夸克胶子和喷注在QGP和冷核中的能相损失机制是什么？色禁闭的强子是怎么由夸克胶子形成的。RHIC p+p和p+A碰撞物理为回答这些问题提供了一个独有的实验场所，也为将来的电子核子对撞机（EIC）做了很多互补和前瞻性测量。其中，格点量子色动力学计算和RHIC实验结果的新闻：“胶子提供了50%的质子自旋”（Gluons Provide Half of the Proton’s Spin）入选2017美国物理十大进展^【[i]^】。