PUB - Publikationen an der Universität Bielefeld

In current times, the study hadronic collisions is a prominent research field in experimental and theoretical physics. Simulations of hadronic collisions are typically based on a hydrodynamic description that relies on equilibration. But accurate depic- tions of the early time period or of collisions with few produced particles may require a non-equilibrium description of the dynamics. In this work we describe equilibrium and non-equilibrium effects in hadronic collisions ranging from small to large systems within model descriptions based on kinetic theory and compare to hydrodynamics. We focus on results for cooling due to longitudinal expansion and radial as well as anisotropic flow. We employ an effective kinetic description, based on the Boltzmann equation in the relaxation time approximation, to study the space-time dynamics and development of transverse flow of small and large collision systems. By combining analytical insights in the small opacity limit with numerical simulations at larger opacities, we are able to describe the development of transverse flow from very small to very large opacities. Suprisingly, we find that deviations between kinetic theory and hydrodynamics persist even in the limit of very large opacities, which can be attributed to the presence of the early pre-equilibrium phase. For decades hydrodynamics has been used as the main tool for simulating heavy ion collisions, being highly successful in describing their phenomenology. However, a priori it is not clear why this description should be as accurate as it is in simulating a system whose size is not necessarily large compared to the mean free path of its constituents, with possibly sizable local fluctuations involving large gradients and a far-from-equilibrium initial state. A rigorous global examination of possible problems of this description has yet to be performed. In this work, we present our results of comparing hydrodynamic and hybrid simulations to kinetic theory in a simplified dynamical model. We point out inaccuracies of hydrodynamical models and present modified setups that can improve them.