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Abstract: The hadronization of an expanding partonic fireball is studied within the Parton-Hadron-String 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 found to be driven by strong gradients in the parton mean-fields. An analysis of the elliptic flow demonstrates a linear correlation with the spatial eccentricity as in the 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 ( or ) 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 grand-canonical partition function at temperature MeV. [Copyright &y& Elsevier]

We investigate hadron production and attenuation of hadrons with strange and charm quarks (or antiquarks) as well as high transverse momentum hadrons in relativistic nucleus–nucleus collisions from 2 A GeV to 21.3 A TeV within two independent transport approaches (UrQMD and HSD). Both transport models are based on quark, diquark, string and hadronic degrees of freedom, but do not include any explicit phase transition to a quark–gluon plasma. From our dynamical calculations we find that both models do not describe the maximum in the K+/π+ ratio at 20–30 A GeV in central Au+Au collisions found experimentally, though the excitation functions of strange mesons are reproduced well in HSD and UrQMD. Furthermore, the transport calculations show that the charmonium recreation by D+&Dmacr;→J/Ψ+meson reactions is comparable to the dissociation by ‘comoving’ mesons at RHIC energies contrary to SPS energies. This leads to the final result that the total J/Ψ suppression as a function of centrality at RHIC should be less than the suppression seen at SPS energies where the ‘comover’ dissociation is substantial and the backward channels play no role. Furthermore, our transport calculations—in comparison to experimental data on transverse momentum spectra from pp, d+Au and Au+Au reactions—show that pre-hadronic effects are responsible for both the hardening of the hadron spectra for low transverse momenta (Cronin effect) as well as the suppression of high pT hadrons. The mutual interactions of formed hadrons are found to be negligible in central Au+Au collisions at √ of s=200 GeV for pT≥6 GeV/c and the sizeable suppression seen experimentally is attributed to a large extent to the interactions of ‘leading’ pre-hadrons with the dense environment. [Copyright &y& Elsevier]

The production and propagation of antikaons—described by dynamical spectral functions Ah(X,P→,M2) as evaluated from a coupled channel G-matrix approach—is studied for nucleus–nucleus collisions at SIS energies in comparison to the conventional quasi-particle limit and the available experimental data using off-shell transport theory. We find that the K− spectra for 12C+12C and 58Ni+58Ni at 1.8 A GeV remain underestimated in the G-matrix approach as in the on-shell quasi-particle approximation whereas the preliminary spectra for Au+Au at 1.5 A GeV are well described in both limits. This also holds approximately for the K− rapidity distributions in semi-central collisions of Ni+Ni at 1.93 A GeV. However, in all limits considered there is no convincing description of all spectra simultaneously. Our off-shell transport calculations, furthermore, demonstrate that the strongest in-medium effects should be found for low antikaon momenta in the center-of-mass frame, since the deceleration of the antikaons in the attractive Coulomb and nuclear potentials and the propagation to the on-shell mass induces a net shift and squeezing of the K− spectra to the low momentum regime. [Copyright &y& Elsevier]

We study the proton sideward and elliptic differential flow for Au+Au collisions at AGS energies (2–8 A GeV) in a microscopic relativistic transport model that includes all baryon resonances up to a mass of 2 GeV as well as string degrees of freedom for the higher hadronic excitations. In order to explore the sensitivity of the various differential flows to the nuclear equation of state (EoS) we use three different parameterizations of the scalar and vector mean-fields, i.e., NL2 (soft), NL23 (medium) and NL3 (hard), with their momentum dependence fitted to the experimental Schro¨dinger equivalent potential (at normal nuclear matter density ρ0) up to kinetic energies of 1 GeV. We calculate the excitation function of sideward and elliptic flow within these parameter sets for Au+Au collisions and compare with the recent data from the E895 Collaboration as a function of rapidity, impact parameter and transverse momentum, respectively. We find that the best description of the differential data is provided by a rather ‘stiff’ EoS at 2 A GeV (NL3) while at higher bombarding energies (4–8 A GeV) a ‘medium’ EoS leads to the lowest χ2 with respect to the data. However, the differences in the transverse and elliptic flows (from the different parameter sets) become of minor significance at 4–8 A GeV. We attribute this insensitivity to a similar reduction of the vector potential in all models and to the dominance of string degrees of freedom at these bombarding energies. [Copyright &y& Elsevier]

The production of antiprotons in p+A reactions is calculated in a microscopic transport approach employing hadronic and string degrees of freedom (HSD). It is found that the abundancies of antiprotons as observed by the E910 Collaboration in p+A reactions at 12.3 GeV/c as well as 17.5 GeV/c can approximately be described on the basis of primary proton–nucleon and secondary meson–baryon production channels for all targets. The transport calculations demonstrate that the antiproton rapidity distributions for heavy targets are sensitive to the p¯ (or hadron) formation time in the nuclear medium. Within our analysis the data from the E910 Collaboration are reasonably described with a formation time of 0.4–0.8 fm/c in the hadron rest frame. [Copyright &y& Elsevier]

In this review we address the dynamics of relativistic heavy-ion reactions and in particular the information obtained from electromagnetic probes that stem from the partonic and hadronic phases. The out-of-equilibrium description of strongly interacting relativistic fields is based on the theory of Kadanoff and Baym. For the modeling of the partonic phase we introduce an effective dynamical quasiparticle model (DQPM) for QCD in equilibrium . In the DQPM, the widths and masses of the dynamical quasiparticles are controlled by transport coefficients that can be compared to the corresponding quantities from lattice QCD. The resulting off-shell transport approach is denoted by Parton–Hadron–String Dynamics (PHSD) and includes covariant dynamical transition rates for hadronization and keeps track of the hadronic interactions in the final phase. It is shown that the PHSD captures the bulk dynamics of heavy-ion collisions from lower SPS to LHC energies and thus provides a solid basis for the evaluation of the electromagnetic emissivity, which is calculated on the basis of the same dynamical parton propagators that are employed for the dynamical evolution of the partonic system. The production of direct photons in elementary processes and heavy-ion reactions is discussed and the present status of the photon v 2 “puzzle”–a large elliptic flow v 2 of the direct photons experimentally observed in heavy-ion collisions–is addressed for nucleus–nucleus reactions at RHIC and LHC energies. The role of hadronic and partonic sources for the photon spectra and the flow coefficients v 2 and v 3 is considered as well as the possibility to subtract the QGP signal from the experimental observables. Furthermore, the production of e + e − or μ + μ − pairs in elementary processes and A + A reactions is addressed. The calculations within the PHSD from SIS to LHC energies show an increase of the low mass dilepton yield essentially due to the in-medium modification of the ρ -meson and at the lowest energy also due to a multiple regeneration of Δ -resonances. Furthermore, pronounced traces of the partonic degrees-of-freedom are found in the intermediate dilepton mass regime ( 1.2 GeV < M < 3 GeV ) at relativistic energies, which will also shed light on the nature of the very early degrees-of-freedom in nucleus–nucleus collisions. [ABSTRACT FROM AUTHOR]