Non-equilibrium QCD in Jet Quenching

The strong interaction describes how the atomic nucleus is built from protons and neutrons, which, in turn, consist of even smaller components, the quarks and gluons. These do not exist as free particles in general, but instead only occur as part of bound particles such as the proton. At very high energies and temperatures, however, the quarks and gluons form a new state of matter, the quark-gluon plasma, in which they behave as free particles. This plasma existed in the early universe shortly after the Big Bang, and is possibly also found inside neutron stars. Experimentally, this state of matter can be produced in large particle colliders where heavy ions are collided. This project aims to study the properties of the quark-gluon plasma in these heavy-ion collisions, in particular its nonequilibrium properties at early times. Using highly energetic particles called jets, properties of this plasma can be determined experimentally by measuring the modifications of these jets in the presence of the plasma. In the theoretical description of jets, their modification can be described through the dependence on a single function of the medium called the elastic collision kernel. However, the theoretical description of the quark-gluon plasma created in heavy-ion collisions constitutes an enormous challenge, since the system is neither weakly nor strongly coupled where theoretical methods exist and are applicable. In this project, therefore, methods for both weakly and strongly coupled systems are used and compared to obtain the elastic collision kernel and study its universal properties. This will enable a better understanding of the strong interaction and its properties in and out of thermal equilibrium.