Direct photon spectra and elliptic flow v2 in heavy-ion collisions at RHIC and LHC energies are investigated within a relativistic transport approach incorporating both hadronic and partonic phases - the Parton-Hadron-String Dynamics (PHSD). The resu
lts suggest that a large v2 of the direct photons - as observed by the PHENIX Collaboration - signals a significant contribution of photons produced in interactions of secondary mesons and baryons in the late stages of the collision. In order to further differentiate the origin of the direct photon azimuthal asymmetry, we compare our predictions for the centrality dependence of the direct photon yield to the recent measurements by the PHENIX Collaboration and provide predictions for Pb+Pb collisions at LHC energies with respect to the direct photon spectra and v2(pT) for 0-40% centrality.
The Parton-Hadron-String-Dynamics (PHSD) transport model is used to study the influence of the initial size of spatial fluctuations of the interacting system on flow observables in Pb-Pb collisions at $sqrt{s_{NN}}$ = 2.76 TeV for different centralit
ies. While the flow coefficients $v_2$, $v_3$, $v_4$ and $v_5$ are reasonably described in comparison to the data from the ALICE Collaboration for different centralities within the default setting, no essential sensitivity is found with respect to the initial size of spatial fluctuations even for very central collisions where the flow coefficients are dominated by the size of initial state fluctuations. We attribute this lack of sensitivity partly to the low interaction rate of the degrees-of-freedom in this very early phase of order $sim$ 0.3 fm/c which is also in common with the weakly interacting color glass condensate (CGC) or glasma approach. Moreover, since the event shape in the transverse plane is approximately the same for different size of spatial fluctuations very similar eccentricities $epsilon_n$ are transformed to roughly the same flow coefficients $v_n$ in momentum space.
Recent STAR data for the directed flow of protons, antiprotons and charged pions obtained within the beam energy scan program are analyzed within the Parton-Hadron-String-Dynamics (PHSD/HSD) transport models. Bo
In this study we investigate the dynamics of strongly interacting parton-hadron matter by calculating the centrality dependence of direct photons produced in Au+Au collisions at $sqrt{s_{NN}}=200$ GeV within the Parton-Hadron-String Dynamics (PHSD) t
ransport approach. As sources for direct photons, we incorporate the interactions of quarks and gluons as well as hadronic interactions ($pi+pitorho+gamma$, $rho+pitopi+gamma$, meson-meson bremsstrahlung $m+mto m+m+gamma$, meson-baryon bremsstrahlung $m+Bto m+B+gamma$), the decays of $phi$ and $a_1$ mesons and the photons produced in the initial hard collisions (pQCD). Our calculations suggest that the channel decomposition of the observed spectrum changes with centrality with an increasing (dominant) contribution of hadronic sources for more peripheral reactions. Furthermore, the thermal photon yield is found to scale roughly with the number of participant nucleons as $N_{part}^alpha$ with $alpha approx$ 1.5, whereas the partonic contribution scales with an exponent $alpha_p approx1.75$. Additionally, we provide predictions for the centrality dependence of the direct photon elliptic flow $v_2(p_T)$. The direct photon $v_2$ is seen to be larger in peripheral collisions compared to the most central ones since the photons from the hot deconfined matter in the early stages of the collision carry a much smaller elliptic flow than those from the final hadronic interactions.
We study the properties of strongly interacting massive quantum fields in space-time as resulting from a parametric decay of the fields with a large decay width $gamma$. The resulting imaginary part of the retarded and advanced propagators in this ca
se is of Lorentzian form and the theory conserves microcausality, i.e. the commutator between the fields vanishes for space-like distances in space-time. However, when considering separately space-like and time-like components of the spectral function in momentum space we find microcausality to be violated for each component separately. This implies that the modeling of effective field theories for strongly interacting systems has to be considered with great care and restrictions to time-like four momenta in case of broad spectral functions have to be ruled out. Furthermore, when employing effective propagators with a width $gamma({bf p}^2)$ depending explicitly on three-momentum ${bf p}$ the commutator of the fields no longer vanishes for $r>t$ since the related field theory becomes nonlocal and violates microcausality.
The dynamics of partons and hadrons in relativistic nucleus-nucleus collisions is analyzed within the novel Parton-Hadron-String Dynamics (PHSD) transport approach, which is based on a dynamical quasiparticle model for the partonic phase (DQPM) inclu
ding a dynamical hadronization scheme. The PHSD approach is applied to nucleus-nucleus collisions from low SPS to LHC energies. The traces of partonic interactions are found in particular in the elliptic flow of hadrons and in their transverse mass spectra. We investigate also the equilibrium properties of strongly-interacting infinite parton-hadron matter characterized by transport coefficients such as shear and bulk viscosities and the electric conductivity in comparison to lattice QCD results.
Dilepton production in $pp$ and $Au+Au$ nucleus-nucleus collisions at $sqrt{s}$ = 200 GeV as well as in $In+In$ and $Pb+Au$ at 158 A$cdot$GeV is studied within the microscopic HSD transport approach. A comparison to the data from the PHENIX Collabora
tion at RHIC shows that standard in-medium effects of the $rho, omega$ vector mesons - compatible with the NA60 data for $In+In$ at 158 A$cdot$GeV and the CERES data for $Pb+Au$ at 158 A$cdot$GeV - do not explain the large enhancement observed in the invariant mass regime from 0.2 to 0.5 GeV in $Au+Au$ collisions at $sqrt{s}$ = 200 GeV relative to $pp$ collisions.
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. A
part from strong parton interactions the expansion and development of collective flow is found to be driven by strong gradients in the parton mean-fields. An analysis of the elliptic flow $v_2$ 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 ($qbar{q}$ 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. Hadronic particle ratios turn out to be in line with those from a grandcanonical partition function at temperature $T approx 170$ MeV.
This review provides a written version of the lectures presented at the Schladming Winter School 2008, Austria, on Nonequilibrium Aspects of Quantum Field Theory. In particular, it shows the way from quantum-field theory - in two-particle irreducible
approximation - to the Kadanoff-Baym (KB) equations and various approximations schemes of the KB equations in phase space. This ultimately leads to the formulation of an off-shell transport theory that well incorporates the underlying quantum physics. Remarkably, these transport equations may be solved within a testparticle representation that allows to study non-equilibrium quantum systems in the weak and strong coupling regime. Actual applications to dilepton production in heavy-ion reactions are presented in comparison with available data. The approach, furthermore, allows to address the hadronization process from partonic to hadronic degrees of freedom.
Dynamical quasiparticle properties are determined from lattice QCD along the line of the Peshier model for the running strong coupling constant in case of three light flavors. By separating time-like and space-like quantities in the number density
and energy density the effective degrees of freedom in the gluon and quark sector may be specified from the time-like densities. The space-like parts of the energy densities are identified with interaction energy (or potential energy) densities. By using the time-like parton densities (or scalar densities) as independent degrees of freedom variations of the potential energy densities with respect to the time-like gluon and/or fermion densities lead to effective mean-fields for time-like gluons and quarks as well as to effective gluon-gluon, quark-gluon and quark-quark (quark-antiquark) interactions. The latter dynamical quantities are found to be approximately independent on the quark chemical potential and thus well suited for an inplementation in off-shell parton transport approaches. Results from the dynamical quasiparticle model (DQPM) in case of two dynamical light quark flavors are compared to lattice QCD calculations for the net quark density as well as for the back-to-back differential dilepton production rate by $q-{bar q}$ annihilation. The DQPM is found to pass the independent tests.
