Search

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]

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 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]

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 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

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]

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]