Search

We study the production of strange hadrons in nucleus-nucleus collisions from 4 to 160 A GeV within the parton-hadron-string dynamics (PHSD) transport approach that is extended to incorporate essentials aspects of chiral symmetry restoration (CSR) in the hadronic sector (via the Schwinger mechanism) on top of the deconfinement phase transition as implemented in PHSD. Especially the K+/π+ and the (Λ + Σ0)/π- ratios in central Au+Au collisions are found to provide information on the relative importance of both transitions. The modeling of chiral symmetry restoration is driven by the pion-nucleon Σ term in the computation of the quark scalar condensate 〈 q¯q〉 that serves as an order parameter for CSR and also scales approximately with the effective quark masses ms and mq . Furthermore, the nucleon scalar density ρs , which also enters the computation of 〈q¯q〉, is evaluated within the nonlinear σ - ω model which is constrained by Dirac-Brueckner calculations and low-energy heavy-ion reactions. The Schwinger mechanism (for string decay) fixes the ratio of strange to light quark production in the hadronic medium. We find that above ∼80 A GeV the reaction dynamics of heavy nuclei is dominantly driven by partonic degrees of freedom such that traces of the chiral symmetry restoration are hard to identify. Our studies support the conjecture of “quarkyonic matter” in heavy-ion collisions from about 5 to 40 A GeV and provide a microscopic explanation for the maximum in the K+/π+ ratio at about 30 A GeV, which only shows up if a transition to partonic degrees of freedom is incorporated in the reaction dynamics and is discarded in the traditional hadron-string models. [ABSTRACT FROM AUTHOR]

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 and HSD) transport models and a 3-fluid hydrodynamics (3FD) approach. Both versions of the kinetic approach, HSD and PHSD, are used to clarify the role of partonic degrees of freedom. The PHSD results, simulating a partonic phase and its coexistence with a hadronic one, are roughly consistent with data. The hydrodynamic results are obtained for two equations of state (EoS), a pure hadronic EoS and an EoS with a crossover type transition. The latter case is favored by the STAR experimental data. Special attention is paid to the description of antiproton directed flow based on the balance of pp- annihilation and the inverse processes for pp- pair creation from multimeson interactions. Generally, the semiqualitative agreement between the measured data and the model results supports the idea of a crossover type of quark-hadron transition that softens the nuclear EoS but shows no indication of a first-order phase transition. [ABSTRACT FROM AUTHOR]

We study the electric conductivity as well as the magnetic response of hot QCD matter at various temperatures T and chemical potentials fiq within the off-shell parton-hadron-string dynamics transport approach for interacting partonic systems in a finite box with periodic boundary conditions. The response of the strongly interacting system ¡n equilibrium to an external electric field defines the electric conductivity σ0, whereas the response to a moderate external magnetic field defines the induced diamagnetic moment μt (T,μq) as well as the spin susceptibility Xs(T,μq). We find a sizable temperature dependence of the dimensionless ratio σ0/T well in line with calculations in a relaxation time approach for Tc < T < 2.5Tc. as well as an increase of σ0 with μq²/T². Furthermore, the frequency dependence of the electric conductivity σ(Ω) shows a simple functional form well in line with results from the dynamical quasiparticle model. The spin susceptibility Xs(T,μq) is found to increase with temperature T and to rise μq²/T² too. The actual values for the magnetic response of the QGP in the temperature range below 250 MeV show that the QGP should respond diamagnetically in actual ultrarelativistic heavy-ion collisions since the maximal magnetic fields created in these collisions are smaller than BC(T), which defines a boundary between diamagnetism and paramagnetism. [ABSTRACT FROM AUTHOR]

We calculate the centrality dependence of direct photons produced in Au+Au collisions at the invariant collision energy √SNN = 200 GeV and their transverse momentum spectra within the parton-hadron-string dynamics transport approach. As sources for "direct" photons, we incorporate the interactions of quarks and gluons as well as hadronic interactions (π + π --> p + y, p + π -> π + y, meson-meson bremsstrahlung in + m --> m + m + y, and meson-baryon bremsstrahlung m + B --> m + B + y), the decays of ø and a1 mesons, and the photons produced in the initial hard collisions. We find that the transverse momentum pT spectra of the "thermal" photons (i.e., the direct photons after the pQCD contribution is subtracted) deviate from exponential distributions and, consequently, observe a strong dependence of the inverse slope parameter Teff on the fitting range in pT. On the other hand, all the obtained "effective temperatures" are well above the critical temperature for the deconfinement phase transition even for peripheral collisions, reflecting primarily a "blue shift" due to radial collective motion of hadrons. 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 Nαpart with α ≈ 1.5, whereas the partonic contribution scales with an exponent ap αp ∾ 1.75. Additionally, we provide predictions for the centrality dependence of the direct photon elliptic flow v2(PT). The photons from the hot deconfined matter in the early stages of the collision carry a much smaller elliptic flow than the final hadrons. Consequently, the direct photon v2 in the most central bin is of the order of a few percent. On the other hand, the elliptic flow of direct photons is considerably larger in more peripheral collisions, approaching that of hadrons. [ABSTRACT FROM AUTHOR]

We study the dynamics of color fields as generated by simple configurations of relativistic particles with Abelian and non-Abelian [SU(2)] charges in the classical limit. We find that chromodynamic (non-Abelian) systems generally show Coulomb-like features by analogy with electrodynamics. A peculiar feature in the non-Abelian case is the additional strength of the chromoelectric and chromomagnetic fields caused by the contribution of changing the color charge. This change of color SU(2) charges results in a rotation of the color vector, which is getting very fast at close partonic distances. The presence of this non-Abelian additional term in the chromoelectric and chromomagnetic fields creates a color charge glow, which is manifested as a distinct color wave disturbance arising due to the finite distance at which the color interaction becomes active. This situation may be relevant to the hadronization phase in ultrarelativistic heavy-ion collisions, where the partonic state is governed by strong local color fluctuations. [ABSTRACT FROM AUTHOR]

We study the electric conductivity of hot QCD matter at various temperatures T within the off-shell parton-hadron-string dynamics transport approach for interacting partonic, hadronic or mixed systems in a finite box with periodic boundary conditions. The response of the strongly interacting system in equilibrium to an external electric field defines the electric conductivity σ0. We find a sizable temperature dependence of the ratio σ0/T well in line with calculations in a relaxation time approach for Tc

We study e+e- pair production in proton-proton and central Pb+Pb collisions at vsNN=2.76 TeV within two models: an extended statistical hadronization model (SHM) and the Parton-Hadron-String Dynamics (PHSD) transport approach. We find that the PHSD calculations roughly agree with the dilepton spectrum from hadronic sources with the "cocktail" estimates from the statistical hadronization model matched to available data at Large Hadron Collider energies. The dynamical simulations within the PHSD show a moderate increase of the low-mass dilepton yield essentially due to the in-medium modification of the ? meson. Furthermore, pronounced traces of the partonic degrees of freedom are found in the PHSD results in the intermediate-mass regime. The dilepton production from the strongly interacting quark-gluon plasma (sQGP) exceeds that from the semileptonic decays of open charm and bottom mesons. Additionally, we observe that a transverse momentum cut of 1 GeV/c further suppresses the relative contribution of the heavy-meson decays to the dilepton yield, such that the sQGP radiation strongly dominates the spectrum for masses from 1 to 3 GeV, allowing a closer look at the electromagnetic emissivity of the partonic plasma in the early phase of Pb+Pb collisions. [ABSTRACT FROM AUTHOR]

We address dilepton production in Au + Au collisions at √sNN = 200 GeV by employing the parton-hadron-string dynamics (PHSD) off-shell transport approach. Within the PHSD, one goes beyond the quasiparticle approximation by solving generalized transport equations on the basis of the off-shell Kadanoff-Baym equations for the Green's functions in the phase-space representation. The approach consistently describes the full evolution 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 (QGP) as well as hadronization, to the subsequent interactions in the hadronic phase. With partons described in the PHSD by the dynamical quasiparticle model (DQPM)--matched to reproduce lattice QCD results in thermodynamic equilibrium--we calculate, in particular, the dilepton radiation from partonic interactions through the reactions q―q → &ggr;*, q―q → &ggr;* + g, and qg → &ggr;*q (―qg → &ggr;*―q) in the early stage of relativistic heavy-ion collisions. By comparing our results to the data from the PHENIX Collaboration, we study the relative importance of different dilepton production mechanisms and point out the regions in phase space where partonic channels are dominant. Furthermore, explicit predictions are presented for dileptons within the acceptance of the STAR detector system and compared to the preliminary data. [ABSTRACT FROM AUTHOR]

The hadron-resonance gas (HRG) approach, used to model hadronic matter at small baryon potentials μB and finite temperature T, is extended to finite and large chemical potentials by introducing interactions between baryons in line with relativistic mean-field theory defining an interacting HRG (IHRG). Using lattice data for μB=0 as well as information on the nuclear equation of state at T=0 we constrain the attractive and repulsive interactions of the IHRG such that it reproduces the lattice equation of state at μB=0 and the nuclear equation of state at T=0 and finite μB. The formulated covariant approach is thermodynamically consistent and allows us to provide further information on the phase boundary between hadronic and partonic phases of strongly interacting matter by assuming constant thermodynamic potentials. [ABSTRACT FROM AUTHOR]

The dynamics of baryon-antibaryon annihilation and reproduction (BB↔3M) is studied within the Parton-Hadron-String Dynamics (PHSD) transport approach for Pb+Pb and Au+Au collisions as a function of centrality from lower Super Proton Synchrotron (SPS) up to Large Hadron Collider (LHC) energies on the basis of the quark rearrangement model. At Relativistic Heavy-Ion Collider (RHIC) energies we find a small net reduction of baryon-antibaryon (BB) pairs while for the LHC energy of √sNN=2.76TeV a small net enhancement is found relative to calculations without annihilation (and reproduction) channels. Accordingly, the sizable difference between data and statistical calculations in Pb+Pb collisions at √sNN=2.76 TeV for proton and antiproton yields [ALICE Collaboration, B. Abelev et al., Phys. Rev. C 88, 044910 (2013)], where a deviation of 2.7 σ was claimed by the ALICE Collaboration, should not be attributed to a net antiproton annihilation. This is in line with the observation that no substantial deviation between the data and statistical hadronization model (SHM) calculations is seen for antihyperons, since according to the PHSD analysis the antihyperons should be modified by the same amount as antiprotons. As the PHSD results for particle ratios are in line with the ALICE data (within error bars) this might point towards a deviation from statistical equilibrium in the hadronization (at least for protons and antiprotons). Furthermore, we find that the BB↔3M reactions are more effective at lower SPS energies where a net suppression for antiprotons and antihyperons up to a factor of 2-2.5 can be extracted from the PHSD calculations for central Au+Au collisions. [ABSTRACT FROM AUTHOR]

The quark rearrangement model for baryon-antibaryon annihilation and reproduction (BB↔3M)--incorporated in the Parton-Hadron-String Dynamics (PHSD) transport approach--is extended to the strangeness sector. A derivation of the transition probabilities for the three-body processes is presented and a strangeness suppression factor for the invariant matrix element squared is introduced to account for the higher mass of the strange quark compared to the light up and down quarks. In simulations of the baryon-antibaryon annihilation and reformation in a box with periodic boundary conditions, we demonstrate that our numerical implementation fulfills detailed balance on a channel-by-channel basis for more than 2000 individual 2↔3 channels. Furthermore, we study central Pb+Pb collisions within PHSD from 11.7AGeV to 158AGeV and investigate the impact of the additionally implemented reaction channels in the strangeness sector. We find that the new reaction channels have a visible impact essentially only on the rapidity spectra of antibaryons. The spectra with the additional channels in the strangeness sector are closer to the experimental data than without for all antihyperons. Due to the chemical redistribution between baryons-antibaryons and mesons we find a slightly larger production of antiprotons thus moderately overestimating the available experimental data. We additionally address the question if the antibaryon spectra (with strangeness) from central heavy-ion reactions at these energies provide further information on the issue of chiral symmetry restoration and deconfinement. However, by comparing transport results with and without partonic phase as well as including and excluding effects from chiral symmetry restoration we find no convincing signals in the strange antibaryon sector for either transition due to the strong final-state interactions. [ABSTRACT FROM AUTHOR]

We study the effect of the chiral symmetry restoration (CSR) on heavy-ion collisions observables in the energy range √sNN=3-20 GeV within the parton-hadron-string dynamics (PHSD) transport approach. The PHSD includes the deconfinement phase transition as well as essential aspects of CSR in the dense and hot hadronic medium, which are incorporated in the Schwinger mechanism for the hadronic particle production. We adopt different parametrizations of the nuclear equation of state from the nonlinear σ−ω model, which enter in the computation of the quark scalar density for the CSR mechanism, in order to estimate the uncertainty in our calculations. For the pion-nucleon Σ term we adopt Σπ≈ 45 MeV, which corresponds to some world average. Our systematic studies show that chiral symmetry restoration plays a crucial role in the description of heavy-ion collisions at √sNN=3-20 GeV, realizing an increase of the hadronic particle production in the strangeness sector with respect to the nonstrange one. We identify particle abundances and rapidity spectra to be suitable probes in order to extract information about CSR, while transverse mass spectra are less sensitive. Our results provide a microscopic explanation for the so-called horn structure in the excitation function of the K+/π+ ratio: The CSR in the hadronic phase produces the steep increase of this particle ratio up to √sNN≈7 GeV, while the drop at higher energies is associated to the appearance of a deconfined partonic medium. Furthermore, the appearance and disappearance of the horn-structure are investigated as functions of the system size and collision centrality. We close this work by an analysis of strangeness production in the (T,μB) plane (as extracted from the PHSD for central Au+Au collisions) and discuss the possibilities to identify a possible critical point in the phase diagram. [ABSTRACT FROM AUTHOR]

The quark susceptibility χq at zero and finite quark chemical potential provides a critical benchmark to determine the quark-gluon-plasma (QGP) degrees of freedom in relation to the results from lattice QCD (lQCD) in addition to the equation of state and transport coefficients. Here we extend the familiar dynamical quasiparticle model (DQPM) to partonic propagators that explicitly depend on the three-momentum with respect to the partonic medium at rest in order to match perturbative QCD (pQCD) at high momenta. Within the extended dynamical quasiparticle model (DQPM*) we reproduce simultaneously the lQCD results for the quark number density and susceptibility and the QGP pressure at zero and finite (but small) chemical potential μq. The shear viscosity η and the electric conductivity σe from the extended quasiparticle model (DQPM*) also turn out to be in close agreement with lattice results for μq=0. The DQPM*, furthermore, allows one to evaluate the momentum p, temperature T, and chemical potential μq dependencies of the partonic degrees of freedom also for larger μq, which are mandatory for transport studies of heavy-ion collisions in the regime 5<√sNN<10 GeV. [ABSTRACT FROM AUTHOR]

For nucleus-nucleus collisions at energies currently available at the BNL Relativistic Heavy Ion Collider (RHIC), we calculate observables in two different transport approaches, i.e., the «-body molecular dynamical model "relativistic quantum molecular dynamics for strongly interacting matter with phase transition or crossover" (RSP) and the two-body parton hadron string dynamics (PHSD), starting out from the same distribution in the initial energy density at the quark gluon plasma (QGP) formation time. The RSP dynamics is based on the Nambu-Jona-Lasinio (NJL) Lagrangian, whereas in PHSD the partons are described by the dynamical quasiparticle model (DQPM). Despite the very different description of the parton properties and their interactions and of the hadronization in both approaches, the final transverse momentum distributions of pions turn out to be quite similar, which is less visible for the strange mesons owing to the large NJL cross sections involved. Our findings can be attributed, in part, to a partial thermalization of the quark degrees of freedom in central Au + Au collisions for both approaches. The rapidity distribution of mesons shows a stronger sensitivity to the nature of the degrees of freedom involved and to their interaction strength in the QGP. [ABSTRACT FROM AUTHOR]

The heavy quark collisional scattering on partons of the quark gluon plasma (QGP) is studied in a quantum chromodynamics medium at finite temperature and chemical potential. We evaluate the effects of finite parton masses and widths, finite temperature T, and quark chemical potential μq on the different elastic cross sections for dynamical quasiparticles (on- and off-shell particles in the QGP medium as described by the dynamical quasiparticle model "DQPM") using the leading order Born diagrams. Our results show clearly the decrease of the qQ and gQ total elastic cross sections when the temperature and the quark chemical potential increase. These effects are amplified for finite μq at temperatures lower than the corresponding critical temperature Tc(μq). Using these cross sections we, furthermore, estimate the energy loss and longitudinal and transverse momentum transfers of a heavy quark propagating in a finite temperature and chemical potential medium. Accordingly, we have shown that the transport properties of heavy quarks are sensitive to the temperature and chemical potential variations. Our results provide some basic ingredients for the study of charm physics in heavy-ion collisions at Beam Energy Scan at RHIC and CBM experiment at FAIR. [ABSTRACT FROM AUTHOR]

We study the heavy-quark scattering on partons of the quark-gluon plasma (QGP), being especially interested in the collisional (elastic) scattering processes of heavy quarks Q on light quarks q and gluons g. We calculate the different cross sections for perturbative partons (massless on-shell particles in the vacuum) and for dynamical quasiparticles (off-shell particles in the QGP medium as described by the dynamical quasiparticles model "DQPM") using the leading-order Born diagrams. Our results show clearly the effect of a finite parton mass and width on the perturbative elastic [q(g)Q→q(g)Q] cross sections which depend on temperature T, energy density ε, the invariant energy vs, and the scattering angle θ. Our detailed comparisons demonstrate that the finite width of the quasiparticles in the DQPM--which encodes the multiple partonic scattering--has little influence on the cross section for qQ→qQ as well as gQ→gQ scattering, except close to thresholds. Thus, when studying the dynamics of energetic heavy quarks in a QGP medium the spectral width of the degrees-of-freedom may be discarded. We have, furthermore, compared the cross sections from the DQPM with corresponding results from hard-thermal-loop (HTL) approaches. The HTL-inspired models--essentially fixing the regulators by elementary vacuum cross sections and decay amplitudes instead of properties of the QGP at finite temperature--provide quite different results, especially with respect to the temperature dependence of the qQ and gQ cross sections (in all settings). Accordingly, the transport properties of heavy quarks will be very different as a function of temperature when compared to DQPM results. [ABSTRACT FROM AUTHOR]

We calculate the shear η(T) and bulk viscosities ζ(T) as well as the electric conductivity σe(T) and heat conductivity ?(T) within the Nambu-Jona-Lasinio model for three flavors as a function of temperature as well as the entropy density s(T), pressure P(T), and speed of sound cs²(T). We compare the results with other models such as the Polyakov-Nambu-Jona-Lasinio (PNJL) model and the dynamical quasiparticle model (DQPM) and confront these results with lattice QCD data whenever available. We find the NJL model to have a limited predictive power for the thermodynamic variables and various transport coefficients above the critical temperature, whereas the PNJL model and DQPM show acceptable results for the quantities of interest. [ABSTRACT FROM AUTHOR]

We study the transverse momentum spectrum and the elliptic flow v2 of photons produced in Au + Au collisions at √sNN = 200 GeV using the parton-hadron-string dynamics (PHSD) transport approach. As sources for photon production, we incorporate the interactions of off-shell quarks and gluons in the strongly interacting quark-gluon plasma (sQGP) [q + q → g + γ and q(q) + g → q(q) + γ], the decays of hadrons (π → γ + γ, η → γ + γ, ω → π + γ,η' → p + γ, φ → η+ γ, a1 → π + γ as well as their interactions (π + π → p + γ, p + π → π + γ, meson-meson bremsstrahlung m + m → m + m + γ). The PHSD calculations reproduce the transverse momentum spectrum, the effective temperature and the elliptic flow v2 of both inclusive and direct photons as measured by the PHENIX Collaboration. The photons produced in the QGP contribute slightly less then 50% to the observed spectrum, but have small v2. We find that the large direct photon v2--comparable to that of hadrons--can be attributed to the intermediate hadronic scattering channels not subtracted from the data when following the same extraction procedure for v2 as in the PHENIX experiment. On the other hand the v2 of direct photons--as evaluated by the weighted average of direct photon channels--gives a lower signal. The difference between the two extraction procedures for the direct photon v2 can be attributed to different definitions for the yield ratio of direct photons to the background photons. The QGP phase causes the strong elliptic flow of photons indirectly, by enhancing the v2 of final hadrons due to the partonic interaction in terms of explicit parton collisions and the mean-field potentials. We also show that the presence of the QGP radiation is manifest in the slope of the direct photon spectrum, leading to an effective slope parameter Teff far above the critical temperature for the deconfinement phase transition. [ABSTRACT FROM AUTHOR]

The azimuthal anisotropies of the collective transverse flow of charged hadrons are investigated in a wide range of heavy-ion collision energies within the microscopic parton-hadron-string dynamics (PHSD) transport approach, which incorporates explicit partonic degrees of freedom in terms of strongly interacting quasiparticles (quarks and gluons) in line with an equation of state from lattice QCD as well as the dynamical hadronization and hadronic collision dynamics in the final reaction phase. The experimentally observed increase of the elliptic flow ν2 of charged hadrons with collision energy is successfully described in terms of the PHSD approach. The PHSD scaling properties of various collective observables are confronted with experimental data as well as with hydrodynamic predictions. The analysis of higher-order harmonics ν3 and ν4 in the azimuthal angular distribution shows a similar tendency of growing deviations between partonic and purely hadronic models with increasing collision energy. This demonstrates that the excitation functions of azimuthal anisotropies reflect the increasing role of quark-gluon degrees of freedom in the early phase of relativistic heavy-ion collisions. Furthermore, the specific variation of the ratio ν4/(ν2)2 with respect to bombarding energy, centrality, and transverse momentum is found to provide valuable information on the underlying dynamics. [ABSTRACT FROM AUTHOR]

The azimuthal anisotropies of the collective transverse flow of hadrons are investigated in a large range of heavy-ion collision energy within the parton-hadron-string dynamics (PHSD) microscopic transport approach which incorporates explicit partonic degrees of freedom in terms of strongly interacting quasiparticles (quarks and gluons) in line with an equation-of-state from lattice QCD as well as dynamical hadronization and hadronic dynamics in the final reaction phase. The experimentally observed increase of the elliptic flow v2 with bombarding energy is successfully described in terms of the PHSD approach in contrast to a variety of other kinetic models based on hadronic interactions. The analysis of higher-order harmonics ν3 and ν4 shows a similar tendency of growing deviations between partonic and purely hadronic models with increasing bombarding energy. This signals that the excitation functions of azimuthal anisotropies provide a sensitive probe for the underling degrees of freedom excited in heavy-ion collisions. [ABSTRACT FROM AUTHOR]

The hadron string dynamics (HSD) model is generalized to include the creation and evolution of retarded electromagnetic fields as well as the influence of the magnetic and electric fields on the quasiparticle propagation. The time-space structure of the fields is analyzed in detail for noncentral Au + Au collisions at √sNN=200 GeV. It is shown that the created magnetic field is highly inhomogeneous, but in the central region of the overlapping nuclei it changes relatively weakly in the transverse direction. For the impact parameter b=10 fm, the maximal magnetic field- perpendicularly to the reaction plane-is obtained of order eBy/mπ2~5 for a very short time ~0.2 fm/c, which roughly corresponds to the time of a maximal overlap of the colliding nuclei. We find that at any time, the location of the maximum in the eBy distribution correlates with that of the energy density of the created particles. In contrast, the electric field distribution, being also highly inhomogeneous, has a minimum in the center of the overlap region. Furthermore, the field characteristics are presented as a function of the collision energy and the centrality of the collisions. To explore the effect of the back reaction of the fields on hadronic observables, a comparison of HSD results with and without fields is exemplified. Our actual calculations show no noticeable influence of the electromagnetic fields-created in heavy-ion collisions-on the effect of the electric charge separation with respect to the reaction plane. [ABSTRACT FROM AUTHOR]

The parton-hadron string dynamics (PHSD) transport model is used to study the impact of the choice of initial degrees of freedom on the final hadronic and electromagnetic observables in Au+Au collisions at vsNN=200 GeV. We find that a nonperturbative system of massive gluons (scenario I) and a system dominated by quarks and antiquarks (scenario II) lead to different hadronic observables when imposing the same initial energy-momentum tensor Tμν(x) just after the passage of the impinging nuclei. In case of the gluonic initial condition the formation of s,s¯ pairs in the quark-gluon plasma (QGP) proceeds rather slowly, such that the antistrange quarks and accordingly the K+ mesons do not achieve chemical equilibrium even in central Au+Au collisions at vsNN=200 GeV. Accordingly, the K+ rapidity distribution is suppressed in the gluonic scenario, and is in conflict with the data from the BRAHMS Collaboration. The proton and antiproton rapidity distributions also disfavor scenario I. Furthermore, a clear suppression of direct photon and dilepton production is found for the pure gluonic initial conditions, which is not so clearly seen in the present photon and dilepton spectra from Au+Au collisions at vsNN=200 GeV due to a large contribution from other channels. It is argued that dilepton spectra in the invariant mass range 1.2

The direct photon spectra and flow (v2, v3) in heavy-ion collisions at CERN Super Proton Synchrotron, BNL Relativistic Heavy Ion Collider, and CERN Large Hadron Collider energies are investigated within a relativistic transport approach incorporating both hadronic and partonic phases, the parton-hadron-string dynamics (PHSD). In the present work, four extensions are introduced compared to our previous calculations: (i) going beyond the soft-photon approximation (SPA) in the calculation of the bremsstrahlung processes meson + meson → meson + meson + γ, (ii) quantifying the suppression owing to the Landau-Pomeranchuk-Migdal (LPM) coherence effect, (iii) adding the additional channels V + N → N + γ and Δ → N + γ, and (iv) providing PHSD calculations for Pb + Pb collisions at √SNN = 2.76 TeV. The first issue extends the applicability of the bremsstrahlung calculations to higher photon energies to understand the relevant sources in the region pτ = 0.5-1.5 GeV, while the LPM correction turns out to be important for pτ < 0.4 GeV in the partonic phase. The results suggest that a large elliptic flow v2 of the direct photons signals a significant contribution of photons produced in interactions of secondary mesons and baryons in the late (hadronic) stage of the heavy-ion collision. To further differentiate the origin of the direct photon azimuthal asymmetry (late hadron interactions vs electromagnetic fields in the initial stage), we provide predictions for the photon spectra, elliptic flow, and triangular flow v3(pτ) of direct photons at different centralities to be tested by the experimental measurements at the LHC energies. Additionally, we illustrate the magnitude of the photon production in the partonic and hadronic phases as functions of time and local energy density. Finally, the "cocktail" method for an estimation of the background photon elliptic flow, which is widely used in the experimental works, is supported by the calculations within the PHSD transport approach. [ABSTRACT FROM AUTHOR]

In this study we evaluate the dynamical collisional energy loss of heavy quarks, their interaction rate, as well as the different transport coefficients (drag and diffusion coefficients, q, etc). We calculate these different quantities for (i) perturbative partons (on-shell particles in the vacuum with fixed and running coupling) and (ii) for dynamical quasi-particles (off-shell particles in the QGP medium at finite temperature T with a running coupling in temperature as described by the dynamical quasiparticles model). We use the perturbative elastic [

The directed flow of identified hadrons is studied within the parton-hadron-string-dynamics (PHSD) approach for the asymmetric system Cu + Au in noncentral collisions at √SNN = 200 GeV. It is emphasized that due to the difference in the number of protons of the colliding nuclei an electric field emerges, which is directed from the heavy to the light nucleus. This strong electric field is only present for about 0.25 fm/c at √SNN = 200 GeV and leads to a splitting of the directed flow v1, for particles with the same mass but opposite electric charges in case of an early presence of charged quarks and antiquarks. The microscopic calculations of the directed flow for π±,K±,p, and p are carried out in the PHSD by taking into account the electromagnetic field induced by the spectators as well as its influence on the hadronic and partonic quasiparticle trajectories. It is shown that the splitting of the directed flow as a function of pseudorapidity η and in particular as a function of the transverse momentum p, provides a direct access to the electromagnetic response of the very early (nonequilibrium) phase of relativistic heavy-ion collisions and allows us to shed light on the presence (and number) of electric charges in this phase. [ABSTRACT FROM AUTHOR]

The interactions of heavy quarks with the partonic environment at finite temperature T and finite quark chemical potential μq are investigated in terms of transport coefficients within the dynamical quasiparticle model (DQPM) designed to reproduce the lattice-QCD (lQCD) results (including the partonic equation of state) in thermodynamic equilibrium. These results are confronted with those of nuclear many-body calculations close to the critical temperature Tc. The hadronic and partonic spatial diffusion coefficients join smoothly and show a pronounced minimum around Tc at μq = 0 as well as at finite μq. Close to and above Tc its absolute value maTches the lQCD calculations for μq = 0. The smooth transition of the heavy-quark transport coefficients from the hadronic to the partonic medium corresponds to a crossover in line with lattice calculations, and differs substantially from perturbative-QCD calculations which show a large discontinuity at Tc. This indicates that in the vicinity of Tc dynamically dressed massive partons should be the effective degrees of freedom in the quark-gluon plasma. [ABSTRACT FROM AUTHOR]

We study the shear and bulk viscosities of partonic and hadronic matter as functions of temperature T within the parton-hadron-string dynamics (PHSD) off-shell transport approach. Dynamical hadronic and partonic systems in equilibrium are studied by the PHSD simulations in a finite box with periodic boundary conditions. The ratio of the shear viscosity to entropy density η(T)/s(T) from PHSD shows a minimum (with a value of about 0.1) close to the critical temperature Tc, while it approaches the perturbative QCD limit at higher temperatures in line with lattice QCD (1QCD) results. For T

We study the kinetic and chemical equilibration in "infinite" parton matter within the parton-hadron-string dynamics off-shell transport approach, which is based on a dynamical quasiparticle model (DQPM) for partons matched to reproduce lattice QCD results--including the partonic equation of state--in thermodynamic equilibrium. The "infinite" parton matter is simulated by a system of quarks and gluons within a cubic box with periodic boundary conditions, at various energy densities, initialized out of kinetic and chemical equilibrium. We investigate the approach of the system to equilibrium and the time scales for the equilibration of different observables. We, furthermore, study particle distributions in the strongly interacting quark-gluon plasma (sQGP) including partonic spectral functions, momentum distributions, abundances of the different parton species, and their fluctuations (scaled variance, skewness, and kurtosis) in equilibrium. We also compare the results of the microscopic calculations with the ansatz of the DQPM. It is found that the results of the transport calculations are in equilibrium well matched by the DQPM for quarks and antiquarks, while the gluon spectral function shows a slightly different shape due to the mass dependence of the gluon width generated by the explicit interactions of partons. The time scales for the relaxation of fluctuation observables are found to be shorter than those for the average values. Furthermore, in the local subsystem, a strong change of the fluctuation observables with the size of the local volume is observed. These fluctuations no longer correspond to those of the full system and are reduced to Poissonian distributions when the volume of the local subsystem becomes small. [ABSTRACT FROM AUTHOR]

In terms of the parton hadron string dynamics (PHSD) approach--including the retarded electromagnetic field--we investigate the role of fluctuations of the correlation function in the azimuthal angle V of charged hadrons that is expected to be a sensitive signal of local strong parity violation. For the early time we consider fluctuations in the position of charged spectators resulting in electromagnetic field fluctuations as well as in the position of participant baryons defining the event plane. For partonic and hadronic phases in intermediate stages of the interaction we study the possible formation of excited matter in electric charge dipole and quadrupole form as generated by fluctuations. The role of the transverse momentum and local charge conservation laws in the observed azimuthal asymmetry is investigated, too. All these above-mentioned effects are incorporated in our analysis based on event-by-event PHSD calculations. Furthermore, the azimuthal angular correlations from Au + Au collisions observed in the recent STAR measurements within the Relativistic Heavy Ion Collider (RHIC) Beam Energy Scan (BES) program are studied. It is shown that the STAR correlation data at the collision energies of √SNN = 7.7 and 11.5 GeV can be reasonably reproduced within the PHSD. At higher energies the model fails to describe the \jr correlation data resulting in an overestimation of the partonic scalar field involved. We conclude that an additional transverse anisotropy fluctuating source is needed which with a comparable strength acts on both in- and out-of-plane components. [ABSTRACT FROM AUTHOR]

In terms of the hadron-string-dynamics (HSD) approach, we investigate the correlation function in the azimuthal angle &PSgr; of charged hadrons that is expected to be sensitive to a signal of local strong parity violation. Our analysis of Au + Au collisions is based on the recent STAR data within the RHIC beam-energy-scan (BES) program. The HSD model reasonably reproduces STAR data for √sNN =7.7 GeV, while there are some deviations from the experiment at the collision energy of 11.5 GeV and an increase of deviations between theory and experiment at √sNN = 39 GeV. For reference, the results for √sNN = 200 GeV are given as well. The role of the retarded electromagnetic field is discussed and a compensation effect for the action of its electric and magnetic components is pointed out. We conclude that the recent RHIC BES data at √sNN = 1.1 and 11.5 GeV can be understood on the hadronic level without involving the idea of a strong parity violation; however, the HSD model fails to reproduce data for √sNN ∼ 40 GeV and above which suggests that at these energies one has to take into account explicit partonic (quark-qluon) degrees of freedom for a proper treatment of the dynamics as well as a coupling of the partons to fluctuating color fields. [ABSTRACT FROM AUTHOR]

Dilepton production in In + In collisions at 158A GeV is studied within the microscopic parton-hadron-string dynamics (PHSD) transport approach that incorporates explicit partonic degrees of freedom and dynamical hadronization as well as the more familiar hadronic dynamics in the final reaction stages. A comparison to the data of the NA60 Collaboration shows that the measured dilepton yield is well described by including the collisional broadening of vector mesons, while simultaneously accounting for the electromagnetic radiation of the strongly coupled quark-gluon plasma (sQGP) via off-shell quark-antiquark annihilation, quark annihilation with additional gluon bremsstrahlung, the gluon-Compton scattering mechanisms. In particular, the spectra in the intermediate mass range (1 GeV ≤ M ≤ 2.5 GeV) are dominated by quark-antiquark annihilation in the nonperturbative QGP. Also, the observed softening of the transverse mass spectra at intermediate masses (1 GeV ≤ M ≤ 2.5 GeV) is approximately reproduced. Furthermore, for dileptons of low masses (M < 0.6 GeV), we find a sizable contribution from the quark annihilation with additional gluon bremsstrahlung, thus providing another possible window for probing the properties of the sQGP. [ABSTRACT FROM AUTHOR]

It is proposed to identify a strong electric field--created during relativistic collisions of asymmetric nuclei--via the observation of pseudorapidity and transverse momentum distributions of hadrons with the same mass but opposite charge. The results of detailed calculations within the parton-hadron string dynamics (PHSD) approach for the charge-dependent directed flow v1 are presented for semicentral Cu+Au collision at √sNN=200 GeV incorporating the inverse Landau-Pomeranchuk-Migdal (iLPM) effect, which accounts for a delay in the electromagnetic interaction with the charged degrees of freedom. By including the iLPM effect, we achieve a reasonable agreement of the PHSD results for the charge splitting in v1(pT) in line with the recent measurements by the STAR Collaboration for Cu+Au collisions at √sNN=200 GeV while an instant appearance and coupling of electric charges at the hard collision vertex overestimates the splitting by about a factor of 10. We predict that the iLPM effect should practically disappear at energies of √sNN≈9 GeV, which should lead to a significantly larger charge splitting of v1 at the future FAIR/NICA facilities. [ABSTRACT FROM AUTHOR]