Study of hot and dense nuclear matter in relativistic heavy ion collisions

当温度升高时，它倾向于将物质分解成更多的元素粒子，因为结合它们的力被热能压倒。 例如，虽然液态水是由松散结合的水分子组成的，但蒸汽是由未结合的水分子组成的气体。同样，当原子核被加热时，原子核会经历相变，并溶解成更多的夸克和胶子。这发生在大约2万亿开尔文的温度。 由此产生的系统称为夸克胶子等离子体，或QGP，是自然界中观察到的最热和最致密的物质，它是有史以来观察到的最“完美的流体”，具有最低的比粘度。 在过去的二十年里，高能核物理界一直致力于研究这种原始物质，通过实验在相对论重离子碰撞中产生它，并通过深入的理论研究来了解它的性质。随着两台相对论重离子对撞机在美国（相对论重离子对撞机）和欧洲（大型强子对撞机）的运行，QGP的实验已经远远超出了发现阶段，进入了精确测量阶段。对这种强相互作用QCD（量子色动力学）多体系统的理论分析是理解强核相互作用的重要实验基础。 在过去的二十年里，我一直在努力从理论上理解这种原始物质。相对论重离子碰撞产生的QGP经历了几个阶段。虽然QGP阶段是最重要的阶段，但理解整个画面对于从实验数据中提取QGP的性质至关重要。多年来，我的团队一直成功地使用从量子场论到流体动力学模拟的广泛理论工具对QGP进行了全面的研究。 在这个资助计划中，我建议使用我们的流体力学和喷流-QGP相互作用模拟工具（这里的“喷流”指的是高能夸克或胶子），为我们分析和预测QGP现象的成功计划添加重要的新物理元素。首先，适当地考虑胶子的束流方向动力学，将极大地改进初始态的计算。 其次，我们将研究令人惊讶的现象，即使是较小的系统所产生的质子-核碰撞表现出的QGP流体的证据，特别是集中在喷流光谱的修改充分考虑到有限尺寸效应。 第三，我们将对强子和电磁信号的非平衡修正进行研究。最后，我们将使用现有的贝叶斯统计工具来确定QGP的性质，如输运系数在一个系统的方式。通过这样一个全面和深入的研究，我们将能够以一种精确和决定性的方式来表征QGP。

When temperature increases, it tends to break up matter into more elemental particles as the forces binding them get overwhelmed by thermal energy. For instance, while liquid water is made of collection of loosely bound water molecules, steam is a gas of unbound water molecules. Likewise, when atomic nucleus is heated, the nucleus undergoes a phase transition and dissolves into more elemental quarks and gluons. This occurs at around two trillion kelvin temperature. The resulting system called Quark-Gluon Plasma, or QGP, is the hottest and the densest matter ever observed in nature and it is the most "perfect fluid'' with the lowest specific viscosity ever observed. In the past two decades, high energy nuclear physics community has been focusing on the study of this primordial matter by experimentally creating it in relativistic heavy ion collisions and understanding its properties through in-depth theoretical investigations. With two relativistic heavy ion colliders in operation in the USA (the Relativistic Heavy Ion Collider) and Europe (the Large Hadron Collider), experiments on QGP has moved on well beyond the discovery phase to the precision measurement phase. Theoretical analysis of this strongly interacting QCD (Quantum Chromodynamics) many-body system is an important testing ground for our understanding of strong nuclear interaction. In the past two decades, I have made a concerted effort to theoretically understand this primordial matter. QGP created in relativistic heavy ion collisions evolves through several stages. Although QGP phase is the most important stage, understanding the whole picture is crucial in extracting the properties of QGP from experimental data. My group has been successfully conducting such a comprehensive study of QGP for many years using wide range of theoretical tools from quantum field theory to hydrodynamic simulations. In this grant proposal, I propose to add important new physics elements to our successful program of analyzing and predicting QGP phenomenology using our hydrodynamics and jet-QGP interaction simulation tools (a "jet" here refers to a high-energy quark or gluon). First, the initial state calculation will be critically improved by properly taking into account the beam-direction dynamics of gluons. Second, we will investigate the surprising phenomenon that even smaller systems created by proton-nucleus collisions exhibit evidence of QGP fluid, in particular focusing on the modification of the jet spectra fully taking into account the finite size effect. Third, we will carry out studies on the non-equilibrium corrections to the hadronic and electromagnetic signals. Lastly, we will use available Bayesian statistics tools to determine the properties of QGP such as transport coefficients in a systematic way. Through such a comprehensive and thorough study, we will be able to characterize QGP in a precise and decisive manner.