arXiv subjects

W. Cassing

Publications and source records attributed to W. Cassing.

At least 55 records · Page 3Linked to original sources

arXiv2012

Dilepton production in proton-proton and Pb+Pb collisions at sqrt(s_NN)=2.76 TeV

We study e^+e^- pair production in proton-proton and central Pb+Pb collisions at sqrt(s_NN)=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 LHC 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 rho-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 semi-leptonic 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.

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arXiv2012

Collective properties of nucleus-nucleus collisions from AGS to LHC energies

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 $v_2$ of charged hadrons with collision energy is successfully described in terms of the PHSD approach. The analysis of higher-order harmonics $v_3$ and $v_4$ 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 $v_4/(v_2)^2$ with respect to bombarding energy, centrality and transverse momentum is found to provide valuable information on the underlying partonic dynamics.

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arXiv2012

Dilepton production from SIS to LHC energies

We study e+e- pair production in proton-proton and in nucleus-nucleus collisions from SIS to LHC energies within the parton-hadron-string dynamics (PHSD) 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. We find a visible in-medium effect in the low mass dilepton sector from dynamical vector-meson spectral functions from SIS to SPS energies whereas at RHIC and LHC energies such medium effects become more moderate. In the intermediate mass regime from 1.1 to 3 GeV pronounced traces of the partonic degrees of freedom are found at SPS and RHIC energies which superseed the hadronic (multi-meson) channels as well as the correlated and uncorrelated semi-leptonic D-meson decays. The dilepton production from the strongly interacting quark gluon plasma (sQGP) becomes already visible at top SPS energies and more pronounced at RHIC and LHC energies.

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arXiv2012

Azimuthal anisotropies for Au+Au collisions in the parton-hadron transient energy range

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 $v_2$ 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 $v_3$ and $v_4$ 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 $v_4/(v_2)^2$ with respect to bombarding energy, centrality and transverse momentum is found to provide valuable information on the underlying dynamics.

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arXiv2012

Theoretical analysis of a possible observation of the chiral magnetic effect in Au + Au collisions within the RHIC beam energy scan program

In terms of the hadron-string-dynamics (HSD) approach we investigate the correlation function in the azimuthal angle $ψ$ 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 $\sqrt{s_{NN}}=$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 $\sqrt{s_{NN}}=$39 GeV. For reference, the results for $\sqrt{s_{NN}}=$ 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 $\sqrt{s_{NN}}=$7.7 and 11.5 GeV can be understood on the hadronic level without involving the idea of a strong parity violation; however, at $\sqrt{s_{NN}}\sim$40 GeV and above one needs to take into consideration explicit partonic (quark-qluon) degrees-of-freedom for a proper treatment of the dynamics.

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arXiv2012

The QGP phase in relativistic heavy-ion collisions

The dynamics of partons, hadrons and strings 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 partons (DQPM) matched to reproduce recent lattice-QCD results - including the partonic equation of state - in thermodynamic equilibrium. The transition from partonic to hadronic degrees of freedom is described by covariant transition rates for the fusion of quark-antiquark pairs or three quarks (antiquarks), respectively, obeying flavor current-conservation, color neutrality as well as energy-momentum conservation. The PHSD approach is applied to nucleus-nucleus collisions from low SIS to RHIC energies. The traces of partonic interactions are found in particular in the elliptic flow of hadrons as well as in their transverse mass spectra.

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arXiv2012

Rise of azimuthal anisotropies as a signature of the Quark-Gluon-Plasma in relativistic heavy-ion collisions

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 $v_2$ 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 $v_3$ and $v_4$ 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.

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arXiv2011

Analysis of dilepton production in Au+Au collisions at sqrt(s_NN)=200 GeV within the Parton-Hadron-String Dynamics (PHSD) transport approach

We address dilepton production in Au+Au collisions at sqrt(s_NN)=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 and 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+qbar->gamma^*, q+qbar->gamma^*+g and q+g->gamma^*+q (qbar+g->gamma^*+qbar) 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.

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arXiv2011

Dilepton production in nucleus-nucleus collisions at top SPS energy within the Parton-Hadron-String Dynamics (PHSD) transport approach

Dilepton production in In+In collisions at 158 AGeV is studied within the microscopic parton-hadron-string dynamics (PHSD) transport approach that incorporates explicit partonic degrees-of-freedom, 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 and 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.

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arXiv2011

Properties of the partonic phase at RHIC within PHSD

The dynamics of partons, hadrons and strings 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 partons (DQPM) matched to reproduce recent lattice-QCD results -- including the partonic equation of state -- in thermodynamic equilibrium. The transition from partonic to hadronic degrees of freedom is described by covariant transition rates for the fusion of quark-antiquark pairs or three quarks (antiquarks), respectively, obeying flavor current-conservation, color neutrality as well as energy-momentum conservation. In order to explore the space-time regions of 'partonic matter' the PHSD approach is applied to nucleus-nucleus collisions from SPS to RHIC energies. Detailed comparisons are presented for hadronic rapidity spectra and transverse mass distributions. The traces of partonic interactions are found in particular in the elliptic flow of hadrons as well as in an approximate quark-number scaling at the top RHIC energy.

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arXiv2011

Electromagnetic field evolution in relativistic heavy-ion collisions

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 non-central Au+Au collisions at $\sqrt{s_{NN}}=$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 $eB_y/m_π^2\sim$5 for a very short time $\sim$ 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 $eB_y$ 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.

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arXiv2011

Dilepton production in p+p, Cu+Cu and Au+Au collisions at 200 AGeV

We study dilepton production in proton-proton, Cu+Cu as well as in Au+Au collisions at the center-of-mass energy 200 GeV per participating nucleon pair within an extended statistical hadronization model. In extension to earlier studies we incorporate transport calculations for an estimate of uncorrelated e+e- -pairs from semileptonic D meson decays. While the invariant mass spectrum of dielectrons is well understood in the p+p collisions, severe discrepancies among different model scenarios based on hadronic degrees of freedom and recent data from the PHENIX Collaboration are found in heavy-ion collisions in the low mass region from 0.15 to 0.6 GeV as well as in the intermediate mass regime from 1.1 to 3 GeV when employing the standard dilepton sources. We investigate, furthermore, the background from correlated dileptons that are not emitted as a pair from a parent hadron but emerge from semileptonic decays of two correlated daughter hadrons. Our calculations suggest a sizeable contribution of such sources in central heavy-ion collisions in the low mass region. However, even the upper limits of our calculations are found to be far below the dilepton mass spectra of the PHENIX Collaboration.

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arXiv2011

Dilepton production from parton interactions in the early stage of relativistic heavy-ion collisions

We address the dilepton production from the parton interactions in the early stage of relativistic heavy-ion collisions within the parton-hadron-string dynamics (PHSD) off-shell transport approach. The description of partons in PHSD is based on the dynamical quasiparticle model (DQPM) matched to reproduce lattice QCD results in thermodynamic equilibrium. According to the DQPM the constituents of the strongly interacting quark-gluon plasma (sQGP) are massive and off-shell quasi-particles (quarks and gluons) with broad spectral functions. In order to address the electromagnetic radiation of the sQGP, we derive off-shell cross sections of q+qbar->gamma*, q+qbar->gamma*+g and q+g->gamma*+q (qbar+g->gamma*+qbar) reactions taking into account the effective propagators for quarks and gluons from the DQPM. Dilepton production in In+In collisions at 158 AGeV and in Au+Au at sqrt(s)=200 GeV is calculated by implementing these off-shell processes into the PHSD transport approach. By comparing to the data of the NA60 and PHENIX Collaborations, we study the relative importance of different dilepton production mechanisms and point out the regions in phase space where partonic channels are dominant.

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arXiv2011

Parton-Hadron-String Dynamics at Relativistic Collider Energies

The novel Parton-Hadron-String Dynamics (PHSD) transport approach is applied to nucleus-nucleus collisions at RHIC energies with respect to differential hadronic spectra in comparison to available data. The PHSD approach is based on a dynamical quasiparticle model for partons (DQPM) matched to reproduce recent lattice-QCD results from the Wuppertal-Budapest group in thermodynamic equilibrium. The transition from partonic to hadronic degrees of freedom is described by covariant transition rates for the fusion of quark-antiquark pairs or three quarks (antiquarks), respectively, obeying flavor current-conservation, color neutrality as well as energy-momentum conservation. Our dynamical studies for heavy-ion collisions at relativistic collider energies are compared to earlier results from the Hadron-String Dynamics (HSD) approach - incorporating no explicit dynamical partonic phase - as well as to experimental data from the STAR, PHENIX, BRAHMS and PHOBOS collaborations for Au+Au collisions at the top RHIC energy of $\sqrt{s}$ = 200 GeV. We find a reasonable reproduction of hadron rapidity distributions and transverse mass spectra and also a fair description of the elliptic flow of charged hadrons as a function of the centrality of the reaction and the transverse momentum $p_T$. Furthermore, an approximate quark-number scaling of the elliptic flow $v_2$ of hadrons is observed in the PHSD results, too.

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arXiv2010

Dilepton production in the strongly interacting quark-gluon plasma

Dilepton production in relativistic heavy-ion collisions is studied within the microscopic Parton-Hadron-String Dynamics (PHSD) transport approach, which is based on a dynamical quasiparticle model (DQPM) matched to reproduce lattice QCD results in thermodynamic equilibrium. A comparison to the data of the NA60 Collaboration for In+In collisions at 158 AGeV shows that the dilepton spectra are well described by the sum of hadronic and partonic sources, if a collisional broadening of vector mesons is taken into account as well as the off-shell quark-antiquark annihilation (q+qbar->l^+l^- and q+qbar->gl^+l^-) in the QGP. In particular, the observed softening of the mT spectra at intermediate masses is reproduced. The data of the PHENIX collaboration on dilepton production in Au+Au collisions at sqrt(s)=200 GeV for masses above 1 GeV are found to be dominated by the contributions of the QGP radiation and the charm meson decays, while the measured spectrum is underestimated in the mass range from 0.2 to 0.6 GeV.

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arXiv2010

Shear viscosity of the Quark-Gluon Plasma from a virial expansion

We calculate the shear viscosity $η$ in the quark-gluon plasma (QGP) phase within a virial expansion approach with particular interest in the ratio of $η$ to the entropy density $s$, i.e. $η/s$. The virial expansion approach allows us to include the interactions between the partons in the deconfined phase and to evaluate the corrections to a single-particle partition function. In the latter approach we start with an effective interaction with parameters fixed to reproduce thermodynamical quantities of QCD such as energy and/or entropy density. We also directly extract the effective coupling $\ga_{\rm V}$ for the determination of $η$. Our numerical results give a ratio $η/s\approx 0.097$ at the critical temperature $T_{\rm c}$, which is very close to the theoretical bound of $1/(4π)$. Furthermore, for temperatures $T\leq 1.8 T_{\rm c}$ the ratio $η/s$ is in the range of the present experimental estimates $0.1-0.3$ at RHIC. When combining our results for $η/s$ in the deconfined phase with those from chiral perturbation theory or the resonance gas model in the confined phase we observe a pronounced minimum of $η/s$ close to the critical temperature $T_{\rm c}$.

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arXiv2010

Signatures of the strongly interacting QGP in relativistic heavy-ion collisions

The transition from hadronic to partonic degrees of freedom in the course of a relativistic heavy-ion collision is described by the microscopic covariant Parton-Hadron-String Dynamics (PHSD) transport approach. Studying Pb+Pb reactions from 40 to 158 AGeV and comparing the PHSD results to those of the Hadron-String Dynamics (HSD) approach without a phase transition to the QGP, we observe that the existence of the partonic phase has a sizable influence on the transverse mass distribution of final kaons due to the repulsive partonic mean fields. Furthermore, we find a significant effect of the QGP on the production of multi-strange antibaryons due to a slightly enhanced ssbar pair production in the partonic phase from massive time-like gluon decay and to a more abundant formation of strange antibaryons in the hadronization process. Another evidence for pre-hadronization dynamics is gained from a study of di-jet correlations in Au+Au collisions at the top RHIC energy of sqrt(S)=200 GeV. Within the HSD transport approach, the reaction of the hadronic medium to the jet energy loss is calculated. In comparison with the data of the STAR, PHOBOS and PHENIX Collaborations differentially in azimuthal angle and pseudorapidity, the HSD results do not show enough suppression for the `away-side' jet. In addition, the HSD results exhibit neither a `Mach-cone' structure for the angular distribution in the away-side jet nor the `ridge' long-range rapidity correlations for the near-side jet as observed by the STAR and PHOBOS Collaborations, thus suggesting a partonic origin of these structures.

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arXiv2010

Jets propagation through a hadron-string medium

Di-jet correlations in nucleus-nucleus collisions are studied within the Hadron-String-Dynamics (HSD) transport approach taking into account the reaction of the medium on the jet energy loss nonperturbatively. A comparison with the STAR and PHENIX data in central Au+Au collisions at the RHIC energy $\sqrt{s}=200$ GeV is performed differentially, i.e. with respect to correlations in azimuthal angle $Δϕ$ and pseudorapidity $Δη$. The HSD results do not show enough suppression for the `away-side' jets in accordance with earlier perturbative studies. Furthermore, the `Mach-cone' structure for the angle distribution in the `away-side' jet as well as `ridge' long rapidity correlations in the `near-side' jet - observed by the STAR and PHOBOS Collaborations - are not seen in the HSD results, thus suggesting a partonic origin.

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