Exploring quark-gluon-plasma dynamics with CPIC simulations

The aim of this project is to simulate the quark-gluon-plasma (QGP) using colored-particle-in-cell (CPIC) simulations. This project is motivated by profound open questions regarding the early dynamics of the QGP as created in heavy ion collisions at RHIC, at CERN LHC, or at the upcoming GSI FAIR that can be addressed with novel simulation techniques. The primary goal of this project is to improve our knowledge about the early dynamics of the QGP, which includes the emergence and evolution of plasma instabilities and their role in the isotropization and thermalization process. For this purpose, a new open source framework for CPIC simulations will be created that will complement or exceed previous numerical approaches, like viscous hydrodynamical simulations, hard loop simulations, or classical Yang-Mills simulations. The framework will be based on the best practices of fifty years of experience with particle-in-cell (PIC) simulations that have been successfully applied to electromagnetic plasmas. In this project, we will extend such simulations by a gauge-independent lattice-discretization and particle description for the SU(3) gauge group of quantum chromodynamics. While the core of the simulation is straightforward to implement, recent advances in electromagnetic PIC simulations, for example an implementation of spin, will lead to fundamentally improved CPIC simulations. Each technological advance will lead directly to important physics questions that can be addressed. In order to achieve this goal, we will build on top of existing expertise on quark-gluon plasma physics and numerical simulations at the Institute for Theoretical Physics at the TU Vienna. This project will utilize the exceptional computational possibilities at the Vienna Scientific Cluster. The anticipated outcome of this project will be unprecedented insight into the formation and evolution of QGP dynamics. The software developed has the potential for spin-offs in other fields.

In this project, we created a new framework for simulating the earliest stages of ultra- relativistic heavy-ion collisions. Such collisions can produce the quark-gluon plasma, which is a state of matter at such high energies that protons and neutrons are melted into their constituents, the quarks and gluons. The properties of the quark-gluon plasma can be experimentally probed at particle accelerators like RHIC, CERN LHC, or the upcoming GSI FAIR facility. Our new framework allows for improved theoretical insight and understanding of the collision process. Before the quark-gluon-plasma is formed, the collision process leads to a so-called Glasma state. This state is characterized by a rapid anisotropic expansion along the beam direction. Previous descriptions of this state used simplifying assumptions and exploited boost- invariance in order to effectively reduce the dynamics to a two-dimensional system. In this setting, the incoming nuclei are assumed to be infinitely thin due to relativistic Lorentz contraction. Our new approach lifts this assumption and allows for a full three-dimensional treatment of the incoming nuclei as well as the produced Glasma. Technically, we achieve this new description by utilizing a simulation technique that has been applied to electromagnetic plasmas since the 1950s, which is known as particle-in-cell simulation. The extension of this technique to particles and fields goverened by the strong nuclear force, also called color force, is the colored-particle-in-cell simulation. It requires a lattice gauge formalism in order to ensure gauge invariance. A parallelized version of our open source code runs on the Vienna Scientific Cluster. One of the surprising results we obtained is that by adding the full three-dimensional dynamics, we we could show that boost-invariance is broken in a way which is qualitatively compatible with experimental results. We also had a breakthrough on a technical level. By deriving new equations of motion we could solve the numerical Cherenkov instability problem of our simulation along a selected direction. This will allow for even more realistic simulations of the earliest times and will lead to a better understanding of the microscopic processes that govern the transition from the Glasma to the quark-gluon plasma.