CAREER: Nuclear Theory for Strongly Interacting Matter

该奖项全部或部分根据2021年美国救援计划法案（公法117-2）资助。大爆炸后的几微秒，宇宙的温度大约是1万亿度。在这样的温度下，质子和中子熔化，它们的组成部分夸克和胶子自由地漫游，形成一种称为夸克-胶子等离子体的特殊物质相。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，它表现得像一种完美的流体，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，仍然是核物理学中的一个突出挑战。PI将开发新工具并扩展现有工具，以在概念上和计算上应对这一挑战。这将需要开发一种新的流体动力学公式，其中包含夸克胶子等离子体的新特性，并建立一个框架，可用于计算研究其在不同温度和密度下的特性。更广泛地说，该项目的结果将有助于揭示具有类似性质的其他形式物质的性质。PI将指导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门为本科生开设的计算物理课程。在炎热和密集环境中，关于物质的最重要的悬而未决的问题之一是绘制强相互作用理论量子色动力学（QCD）的相图。特别是，由于渐近自由，我们知道，在极高的温度下，核子“熔化”并形成一个新的相，称为夸克-胶子等离子体，其中夸克和胶子不再被限制在核子内。与此同时，这种转变在相图中发生的位置及其性质仍然难以捉摸。格点QCD的适用性，唯一的第一原理方法研究这种相变在强耦合制度，是严重限制了臭名昭著的“符号问题”，野生取消之间的复杂权重的字段配置QCD路径积分使其计算棘手。本研究的目的是：i）通过以避免大相位振荡的方式复杂化QCD路径积分，发展一种新的解决符号问题的框架，目标是使QCD相图可用于第一原理计算，和ii）通过将相对论流体力学中的波动结合起来，理解夸克-胶子等离子体的非平衡性质，该奖项反映了NSF的法定使命，并通过使用基金会的知识价值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.