CAREER: Nuclear Theory for Strongly Interacting Matter

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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.