CAREER: Nuclear Theory for Strongly Interacting Matter

该奖项的全部或部分资金来自《2021年美国救援计划法案》(公法117-2)。在大爆炸后的几微秒内，宇宙的温度大约是1万亿度。在这些温度下，质子和中子融化，它们的成分夸克和胶子自由漫游，形成一种特殊的物质相，称为夸克-胶子等离子体。今天，人们可以在重离子碰撞实验中重现这种夸克-胶子等离子体的微小液滴，并观察到它令人惊讶的特性。例如，它的行为就像一种完美的流体，而不是夸克和胶子的气体。与此同时，了解夸克-胶子等离子体的性质，以及它如何在早期宇宙冷却时转变为质子和中子，仍然是核物理中的一个突出挑战。PI将开发新的工具并扩展现有工具，以从概念和计算上解决这一挑战。这将需要开发一种新的流体动力学公式，其中包括夸克-胶子等离子体的新性质，并建立一个框架，可以用于计算研究其在不同温度和密度下的性质。更广泛地说，这个项目的结果将有助于阐明具有类似性质的其他形式的物质的性质。PI将指导将积极参与研究的研究生和本科生。教育活动还将包括一个专为本科生开设的计算物理课程。关于炎热和稠密环境中的物质，最重要的悬而未决的问题之一是绘制量子色动力学(QCD)的相图，这是强相互作用理论。特别是，由于渐近自由，我们知道在极高的温度下，核子“熔化”并形成一种称为夸克-胶子等离子体的新相，其中夸克和胶子不再被限制在核子内。同时，这种转变发生在相图的什么地方，它的性质仍然难以捉摸。晶格QCD是研究强耦合区这一相变的唯一第一原理方法，它的适用性受到臭名昭著的“符号问题”的严重限制，在这个问题中，QCD路径积分中场组态的复数权重之间的狂野抵消使其计算变得困难。这项研究的目的是i)开发一个新的解决符号问题的框架，目标是通过使QCD路径积分复杂化以避免大的相振荡，以及ii)通过结合相对论流体力学中的涨落来理解夸克-胶子等离子体的非平衡性质，并特别强调在相图中寻找推测的临界点，目标是使QCD相图可用于第一性原理计算。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

This award is funded in whole or in part under the America Rescue Plan Act of 2021 (Public Law 117-2). Microseconds after the Big Bang, the Universe's temperature was roughly a trillion degrees. At these temperatures, protons and neutrons melt, and their constituents, quarks and gluons, roam freely, forming a peculiar phase of matter called quark-gluon plasma. Today, one can recreate tiny droplets of this quark-gluon plasma in heavy ion collision experiments and observe its surprising properties. For example, it behaves like a perfect fluid as opposed to a gas of quarks and gluons. At the same time, understanding the properties of quark-gluon plasma and how it transitioned into protons and neutrons as the early Universe cooled down, is still an outstanding challenge in nuclear physics. The PI will develop new tools and extend existing ones to tackle this challenge conceptually and computationally. This will entail developing a new formulation of fluid dynamics that incorporates the novel properties of quark-gluon plasma and building a framework that can be used to computationally study its properties at different temperatures and densities. More broadly, the results of this project will help shed light on properties of other forms of matter with similar properties. The PI will mentor graduate and undergraduate students who will actively participate in the research. The educational activities will also include a computational physics program dedicated to undergraduates.One of the most important outstanding questions regarding matter at hot and dense environments is mapping the phase diagram of Quantum-Chromo Dynamics (QCD), the theory of strong interactions. In particular, due to asymptotic freedom, we know that at extremely high temperatures, nucleons "melt" and form a new phase known as the quark-gluon plasma where quarks and gluons are no longer confined within nucleons. At the same time where this transition happens in the phase diagram and its properties remains elusive. The applicability of lattice QCD, the only first-principle method of studying this phase transition in the strongly coupled regime, is severely limited by the infamous “sign problem” where wild cancellations between the complex weights of field configurations in the QCD path integral render its computation intractable. This research aims to i) develop a new framework of tackling the sign problem with the goal of making the QCD phase diagram accessible to first-principles computations, by complexifying the QCD path integral in a way that avoids the large phase oscillations, and ii) understanding the out-of-equilibrium properties of the quark-gluon plasma by incorporating fluctuations in relativistic hydrodynamics, with a particular emphasis on the search for the conjectured critical point in the phase diagram.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.