Emissivity and conductivity of parton-hadron matter
We investigate the properties of the QCD matter across the deconfinement phase transition. In the scope of the parton-hadron string dynamics (PHSD) transport approach, we study the strongly interacting matter in equilibrium as well as the out-of equilibrium dynamics of relativistic heavy-ion collisions. We present here in particular the results on the electromagnetic radiation, i.e. photon and dilepton production, in rel- ativistic heavy-ion collisions and the relevant correlator in equilibrium, i.e. the electric conductivity. By comparing our calculations for the heavy-ion collisions to the available data, we determine the relative importance of the various production sources and address the possible origin of the observed strong elliptic flow v2 of direct photons. The electromagnetic emissivity of strongly interacting matter is a subject of longstanding interest and is explored in particular in relativistic nucleus-nucleus collisions, where the photons (and dilep- tons) measured experimentally provide a time-integrated picture of the collision dynamics. The recent observation by the PHENIX Collaboration (1) that the ellipticflow v2(pT) of 'direct photons' produced in minimal bias Au+Au collisions at √ sNN = 200 GeV is comparable to that of the produced pions was a surprise and in contrast to the theoretical expectations and predictions (2-6). We have investi- gated the spectra and elliptic flow of dileptons and photons from relativistic heavy-ion collisions in the scope of the covariant transport approach Parton-Hadron String Dynamics (PHSD) in Refs. (7-11). The PHSD model (12, 13) is an off-shell transport approach that consistently describes the full evolu- tion of a relativistic heavy-ion collision from the initial hard scatterings and string formation through the dynamical deconfinement phase transition to the quark-gluon plasma as well as hadronization and the subsequent interactions in the hadronic phase. The two-particle correlations resulting from the finite width of the parton spectral functions are taken into account dynamically in the PHSD by means of the generalized off-shell transport equations (14) that go beyond the mean field or Boltzmann ap- proximation (15). The transport theoretical description of quarks and gluons in the PHSD is based on the Dynamical Quasi-Particle Model (DQPM) for partons that is constructed to reproduce lattice QCD (lQCD) results for the entropy density, energy density and pressure as functions of temperature for the quark-gluon plasma in thermodynamic equilibrium. In the hadronic sector, PHSD is equivalent to the Hadron-String Dynamics (HSD) approach (16-18). For details about the DQPM model and the off-shell transport approach we refer the reader to the review Ref. (15).