Nonperturbative properties of evolving gluonic plasmas

Quantum Chromodynamics (QCD) is one of the four fundamental forces in the Standard Model of particle physics and among the most important building blocks of modern physics. It describes the interactions among quarks and gluons, which form nuclear matter (protons, neutrons and other hadrons) or, at extremely high temperatures of around 1012 C, a quark-gluon plasma. This extreme state of matter has likely existed in the earliest instants of our universe after the Big Bang. On Earth, the quark-gluon plasma is created in ultra-relativistic heavy-ion collision experiments at large laboratory facilities like the Large Hadron Collider (LHC) and the Relativistic Heavy Ion Collider (RHIC) at early times after the collision. In these experiments, the plasma behaves like a nearly perfect fluid after a very short time scale of the order of a few yoctoseconds ~1 fm/c 3 10-24 s. Right after the collision, created quarks and gluons do not have the same energy distribution as in thermal equilibrium, and their non-equilibrium states can have very different properties. Understanding them at the initial stages that lead to the fluid-like behavior of the quark-gluon plasma constitutes a bottleneck in the analysis of heavy-ion collisions, from which description uncertainties and errors can propagate. Detailed knowledge of the early-time properties is not only necessary, but also opens new opportunities. For instance, early-time dynamics can influence transport coefficients. These are numbers that encode properties of the current state of the plasma and are core ingredients in phenomenological studies of experimental observables like heavy quarks or highly energetic particles (jets) in the plasma. This can lead to unique signatures of the early-time dynamics in heavy-ion collisions. Despite its significance, much is unknown about the non- equilibrium plasma. The goal of this research project is to gain a much deeper understanding of the pre-equilibrium dynamics of the quark-gluon plasma using nonperturbative simulation techniques and to find experimentally testable signatures of the early-time dynamics. By performing large-scale classical- statistical lattice simulations with large gluonic particle numbers, we focus on yet insufficiently understood nonperturbative properties of the plasma. Such include spectral information on quasiparticles in the plasma, gluon dynamics at large length scales, and the impact of the non- equilibrium plasma on transport coefficients and other observables. Moreover, the plasma exhibits universal self-similar dynamics when studied in extreme far-from-equilibrium conditions, and it has been conjectured to form a universality class with scalar field theories. In the project, we also want to scrutinize and to improve our understanding of this universality. This opens interdisciplinary opportunities linking heavy-ion collisions to ultra-cold atom experiments and to reheating after cosmological inflation.

How does matter behave immediately after atomic nuclei collide at nearly the speed of light? This project opened new ways to study these first moments. It shows how energetic particles can act as messengers from the early stages of the collision, and achieved a breakthrough in simulating the strong force in real time. In such collisions, an extremely hot state of matter is created: the quark-gluon plasma. Similar conditions existed in the earliest instants after the Big Bang. The quark-gluon plasma is governed by the strong force, which binds quarks and gluons inside protons, neutrons and atomic nuclei. In the laboratory, this matter exists only for an extremely short time and cannot be observed directly. Researchers therefore reconstruct its properties from particles that reach the detectors. A central result of this project is that very heavy or very energetic particles and particle sprays can carry information from the earliest stages. These messengers, known as hard probes, pass through the emerging medium and are changed by it. The project showed how the first, highly non-equilibrium phases after the collision can leave traces in their later evolution. This creates a promising route for connecting theoretical descriptions of the early plasma with experimental observations. It may become particularly relevant for future data sets at CERN, including high-luminosity LHC operation and collisions of light ions such as oxygen. The project also deepened our understanding of universal behaviour far from equilibrium. Here, "universal" means that very different physical systems can follow similar patterns under extreme conditions. The project demonstrated such patterns in gluon-rich matter in heavy-ion collisions, including self-similar evolution linked to collective structures in the gluon field. It also proposed and demonstrated limiting attractors, which constrain the paths along which non-equilibrium matter can evolve. In related studies of simpler theories, the project identified wave-like excitations as the mechanism driving condensate formation. These results help explain why systems that look very different can nevertheless share common dynamics. Another important success was methodological and computational. The project significantly advanced complex Langevin simulations, a method aimed at one of the hardest problems in theoretical physics: calculating the real-time behaviour of quantum fields, especially those describing the strong force. By combining new simulation techniques developed in this project with recent advances, the team produced the first stable and reliable simulations of a three-dimensional real-time gauge theory and showed in one example how such simulations can be used to calculate real-time correlations. In the long term, these advances help us understand the strong force that binds atomic nuclei and shaped matter in the early universe. They also provide new tools for interpreting high-energy nuclear experiments and for studying real-time quantum dynamics more broadly.