Christian Wesp

Elliptic flow and nuclear modification factor in ultrarelativistic heavy-ion collisions within a partonic transport model.

The quark gluon plasma produced in ultrarelativistic heavy-ion collisions exhibits remarkable features. It behaves like a nearly perfect liquid with a small shear viscosity to entropy density ratio and leads to the quenching of highly energetic particles. We show that both effects can be understood for the first time within one common framework. Employing the parton cascade Boltzmann approach to multiparton scatterings, the microscopic interactions and the space-time evolution of the quark gluon plasma are calculated by solving the relativistic Boltzmann equation. Based on cross sections obtained from perturbative QCD with explicitly taking the running coupling into account, we calculate the nuclear modification factor and elliptic flow in ultrarelativistic heavy-ion collisions. With only one single parameter associated with coherence effects of medium-induced gluon radiation, the experimental data of both observables can be understood on a microscopic level. Furthermore, we show that perturbative QCD interactions with a running coupling lead to a sufficiently small shear viscosity to entropy density ratio of the quark gluon plasma, which provides a microscopic explanation for the observations stated by hydrodynamic calculations.

Calculation of shear viscosity using Green-Kubo relations within a parton cascade

The shear viscosity of a gluon gas is calculated using the Green-Kubo relation. Time correlations of the energy-momentum tensor in thermal equilibrium are extracted from microscopic simulations using a parton cascade solving various Boltzmann collision processes. We nd that the pQCD based gluon bremsstrahlung described by Gunion-Bertsch processes signicantly lowers the shear viscosity by a factor of 3 8 compared to elastic scatterings. The shear viscosity scales with the coupling as 1=( 2 log(1= s)). For constant s the shear viscosity to entropy density ratio =s has no dependence on temperature. Replacing the pQCD-based collision angle distribution of binary scatterings by an isotropic form decreases the shear viscosity by a factor of 3.

Dynamics of the Chiral Phase Transition

The intention of this study is the search for signatures of the chiral phase transition in heavy-ion collisions. To investigate the impact of fluctuations, e.g., of the baryon number, at the transition or at a critical point, the linear sigma model is treated in a dynamical (3+1)-dimensional numerical simulation. Chiral fields are approximated as classical mean fields, and quarks are described as quasi particles in a Vlasov equation. Additional dynamics is implemented by quark-quark and quark-sigma-field interactions. For a consistent description of field-particle interactions, a new Monte-Carlo-Langevin-like formalism has been developed and is discussed.

Mach Cones in Viscous Matter

Employing a microscopic transport model we investigate the evolution of high energetic jets moving through a viscous medium. For the scenario of an unstoppable jet we observe a clearly strong collective behavior for a low dissipative system η/s ≈ 0.005, leading to the observation of cone-like structures. Increasing the dissipation of the system to η/s ≈ 0.32 the Mach Cone structure vanishes. Furthermore, we investigate jet-associated particle correlations. A double-peak structure, as observed in experimental data, is even for low-dissipative systems not supported, because of the large influence of the head shock.

Monte-Carlo approach to particle-field interactions and the kinetics of the chiral phase transition

The kinetics of the chiral phase transition is studied within a linear quark-meson-

Nonequilibrium dynamics and transport near the chiral phase transition of a quark-meson model

\sigma

Shear viscosity of an ultrarelativistic Boltzmann gas with isotropic inelastic scattering processes

model, using a Monte-Carlo approach to semiclassical particle-field dynamics. The meson fields are described on the mean-field level and quarks and antiquarks as ensembles of test particles. Collisions between quarks and antiquarks as well as the

Collective Flow and Mach Cones with transport

q\overline{q}

Kinetics of the chiral phase transition in a linear \(\sigma\) model

annihilation to

Kinetics of the Chiral Phase Transition

\sigma