Simulating the earliest stages of heavy-ion collisions

Relativistic heavy-ion collisions can probe matter under extreme conditions where the constituents of protons and neutrons, the quarks and gluons, are melted into a quark-gluon plasma. This state of matter filled the early universe within microseconds after the Big Bang. Nowadays, it can be produced in particle colliders such as the Large Hadron Collider (LHC) at CERN or the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory. The quark-gluon plasma can only be detected indirectly by its decay products. Therefore, a good theoretical understanding is necessary in order to properly interpret experimental findings. One of the surprising discoveries so far was that the quark-gluon plasma behaves almost like a perfect fluid. Therefore, once it has been created, its evolution can be well described by hydrodynamic equations. Unfortunately, these equations are not applicable to the collision process itself. Its dynamics is governed by the strong nuclear force which can in principle be described by quantum chromodynamics. The strongly interacting nature of this many-particle quantum system renders accurate calculations highly challenging. The aim of this project is to improve our theoretical understanding of the earliest stages of the collision of heavy ions. We have created a simulation framework which can simulate the collision process before a hydrodynamic description of the quark-gluon plasma becomes applicable. This is achieved using a colored particle-in-cell approach in which the strong color fields are discretized on a space-time lattice. At ultrarelativistic energies, the heavy ions are Lorentz contracted to thin discs. In traditional approaches, this is utilized to simplify some of the equations and to only simulate the transverse dynamics. With our approach, we can resolve the full longitudinal dynamics of the collision. This comes with very high computational costs such that our simulations have to be performed on the Vienna Scientific Cluster (VSC). With our simulations, we want to systematically study the influence of the earliest stages on various observables of heavy-ion collisions. In particular, we want to connect our three-dimensional simulation results of the earliest stages to the later hydrodynamical stage of the quark-gluon plasma, which in turn can be linked to the distribution of particles that are detected in the corresponding experiments. We also intend to study the influence of the early time dynamics on extremely high- energetic particles that can be created during the collision process. They can be detected in the form of so-called jets in particle detectors. Finally, we want to deepen our understanding of the effect of quantum vacuum fluctuations on the creation process. With our project, we ultimately want to gain deeper insight into the microscopic nature of isotropization and thermalization in the formation of the quark-gluon plasma.