Publications

The hadronization of an expanding partonic fireball is studied within the Parton-Hadron-Strings Dynamics (PHSD) approach which is based on a dynamical quasiparticle model (DQPM) matched to reproduce lattice QCD results in thermodynamic equilibrium. Apart from strong parton interactions the expansion and development of collective flow is driven by strong gradients in the parton mean-fields. An analysis of the elliptic flow v2 demonstrates a linear correlation with the spatial eccentricity epsilon as in case of ideal hydrodynamics. The hadronization occurs by quark-antiquark fusion or 3 quark/3 antiquark recombination which is described by covariant transition rates. Since the dynamical quarks become very massive, the formed resonant pre-hadronic color-dipole states (q-qbar or qqq) are of high invariant mass, too, and sequentially decay to the groundstate meson and baryon octets increasing the total entropy. This solves the entropy problem in hadronization in a natural way. The resulting particle ratios turn out to be in line with those from a grandcanonical partition function at temperature T approximately 170 MeV, rather independent from the initial temperature, and indicate an approximate strangeness equilibration.

The dynamics of partons, hadrons and strings in relativistic nucleus-nucleus collisions is analyzed within the Parton-Hadron-String Dynamics (PHSD) transport approach, which is based on a Dynamical QuasiParticle Model (DQPM) for partons matched to reproduce lattice-QCD results in thermodynamic equilibrium. Scalar- and vector-interaction densities are extracted from the DQPM as well as effective scalar- and vector-mean fields for the partons. The transition from partonic to hadronic degrees of freedom is described by covariant transition rates for the fusion of quark-antiquark pairs or three quarks (antiquarks), obeying flavor current-conservation, color neutrality and energy-momentum conservation. Since the dynamical quarks and antiquarks become very massive close to the phase transition, the formed resonant pre-hadronic color-dipole states (q-qbar or qqq) are of high invariant mass, too, and sequentially decay to the groundstate meson and baryon octets increasing the total entropy. The PHSD approach is applied to nucleus-nucleus collisions from 20 to 160 A GeV to explore partonic matter and shows a sizeable influence on transverse-mass distributions of kaons and on multi-strange antibaryon production.

We extend the Parton-Hadron-String Dynamics (PHSD) transport approach in the partonic sector by explicitly calculating the total and differential partonic scattering cross sections as a function of temperature T and baryon chemical potential \mu_B on the basis of the effective propagators and couplings from the Dynamical QuasiParticle Model (DQPM) that is matched to reproduce the equation of state of the partonic system above the deconfinement temperature T_c from lattice QCD. The ratio of shear viscosity \eta over entropy density s, i.e. \eta/s, is evaluated using the collisional widths and compared to lQCD calculations for \mu_B = 0 as well. We find only a very modest change of \eta/s with the baryon chemical \mu_B. This also holds for a variety of hadronic observables from central A+A and C+Au collisions in the energy range 5 GeV \leq \sqrt{s_{NN}} \leq 200 GeV when implementing the differential cross sections into the PHSD approach. We only observe small differences in the strangeness and antibaryon sector with practically no sensitivity of rapidity and p_T distributions to the \mu_B dependence of the partonic cross sections. Since we find only small traces of a \mu_B-dependence in heavy-ion observables - although the effective partonic masses and widths as well as their partonic cross sections clearly depend on \mu_B - this implies that one needs a sizable partonic density and large space-time QGP volume to explore the dynamics in the partonic phase. These conditions are only fulfilled at high bombarding energies where \mu_B is, however, rather low. On the other hand, when decreasing the bombarding energy and thus increasing \mu_B, the hadronic phase becomes dominant and accordingly, it will be difficult to extract signals from the partonic dynamics based on "bulk" observables.