PUB - Publikationen an der Universität Bielefeld

It is well established that high-energy heavy-ion collision produce strongly interacting matter under extreme conditions similar to those at the beginning of the universe. One of these states of matter is known under the name quark-gluon plasma (QGP) and is one of the most unique states we are able to access. Initially far from equilibrium, it will eventually thermalize and transition to confined matter. Until today we have learned many things regarding the QGP but understanding it in its full glory is a challenging task and the need for observables that probe the QGP directly is ubiquitous.

Over the years, many probes have been established and although most of them rely on understanding their interaction with the medium, as they participate in the strong force, there are two probes that are unique in the sense that they do not re-interact with the medium after their emission: these probes are photons and dileptons, also referred to as electromagnetic probes. Additionally, photons and dileptons are produced throughout the entire space-time evolution of a heavy-ion collision. Thus, understanding them in the context of a thermalizing QGP allows for comparison to well-known other stages and determination of properties of the pre-equilibrium phase of the QGP.

In order to simulate this pre-equilibrium phase, we will use an effective kinetic QCD (quantum chromodynamics) description and determine the dynamics of quarks and gluons, which in turn will serve as an input in the calculations of the pre-equilibrium transverse momentum spectrum of photons and invariant mass spectrum of dileptons. Based on the explicit forms of the spectra, we further derive universal scaling functions for the respective spectra in terms of the specific shear viscosity $\eta/s$ and entropy density $dS/d\eta \sim {\scriptstyle \left(T\tau^{1/3}\right)_\infty^{3/2}}$ (as well as the equilibrium rate $\tilde C$_*γ*^*Ideal* in the case of photons). The scaling will allow us to make phenomenological comparisons to other sources of electromagnetic radiation during the evolution of a heavy-ion collision and our publicly available implementation allows for an event-by-event calculation of the respective spectra.