arXiv subjects

Wolfgang Cassing

Publications and source records attributed to Wolfgang Cassing.

35 records · Page 2Linked to original sources

arXiv2014

Transport properties of the hot and dense sQGP

The transport properties of the quark gluon plasma (QGP) are studied in a QCD medium at finite temperature and chemical potential. We calculate the shear viscosity $η(T,μ_q)$ and the electric conductivity $σ_e(T,μ_q)$ for a system of interacting massive and broad quasi-particles as described by the dynamical quasi-particle model "DQPM" at finite temperature $T$ and quark chemical potential $μ_q$ within the relaxation time approximation. Our results are in a good agreement with lattice QCD at finite temperature and show clearly the increase of the transport coefficients with increasing $T$ and $μ_q$. Our results provide the basic ingredients for the study of the hot and dense matter in the Beam Energy Scan (BES) at RHIC and CBM at FAIR.

hep-ph↗

arXiv2014

Dynamical collisional energy loss and transport properties of on- and off-shell heavy quarks in vacuum and in the Quark Gluon Plasma

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, $\hat{q}$, 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 quasi-particles model). We use the perturbative elastic $(q(g) Q \rightarrow q (g) Q)$ cross section for the first case, and the Infrared Enhanced Hard Thermal Loop cross sections for the second. The results obtained in this work demonstrate the effects of a finite parton mass and width on the heavy quark transport properties and provide the basic ingredients for an explicit study of the microscopic dynamics of heavy flavors in the QGP - as formed in relativistic heavy-ion collisions - within transport approaches developed previously by the authors.

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arXiv2013

Transport coefficients from the Nambu-Jona-Lasinio model for $SU(3)_f$

We calculate the shear $η(T)$ and bulk viscosities $ζ(T)$ as well as the electric conductivity $σ_e(T)$ and heat conductivity $κ(T)$ within the Nambu-Jona-Lasinio model for 3 flavors as a function of temperature as well as the entropy density $s(T)$, pressure $P(T)$ and speed of sound $c_s^2(T)$. 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.

hep-ph↗

arXiv2011

Dilepton production in proton-proton collisions at top RHIC energy

We study dielectron production in proton-proton collisions at top RHIC beam energy within an extended statistical hadronization model. The invariant mass spectrum of correlated dielectron pairs is evaluated in the low invariant mass region and calculated results are compared with the PHENIX experiment. The model is found to be able to describe the data very well up to invariant masses of 1 GeV with few adjustable parameters.

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arXiv2010

Dynamical equilibration in strongly-interacting parton-hadron matter

We study the kinetic and chemical equilibration in 'infinite' parton-hadron matter within the Parton-Hadron-String Dynamics transport approach, which is based on a dynamical quasiparticle model for partons matched to reproduce lattice-QCD results - including the partonic equation of state - in thermodynamic equilibrium. The 'infinite' matter is simulated within a cubic box with periodic boundary conditions initialized at different baryon density (or chemical potential) and energy density. The transition from initially pure partonic matter to hadronic degrees of freedom (or vice versa) occurs dynamically by interactions. Different thermodynamical distributions of the strongly-interacting quark-gluon plasma (sQGP) are addressed and discussed.

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arXiv2007

Charmed signatures for phase transitions in heavy-ion collisions

The interplay of charmonium production and suppression in In+In and Pb+Pb reactions at 158 AGeV and in Au+Au reactions at sqrt(s)=200 GeV is investigated with the HSD transport approach within the `hadronic comover model' and the `QGP melting scenario'. The results for the J/Psi suppression and the Psi' to J/Psi ratio are compared to the recent data of the NA50, NA60, and PHENIX Collaborations. We find that, at 158 AGeV, the comover absorption model performs better than the scenario of abrupt threshold melting. However, neither interaction with hadrons alone nor simple color screening satisfactory describes the data at sqrt(s)=200 GeV. A deconfined phase is clearly reached at RHIC, but a theory having the relevant degrees of freedom in this regime (strongly interacting quarks/gluons) is needed to study its transport properties.

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arXiv2006

Charmonium dissociation by mesons in heavy-ion collisions

The charmonium dissociation by mesons in relativistic heavy-ion reactions is analyzed within the Regge approach. It is shown that the inclusion of the initial and final state interactions in the dissociation of J/Psi to bar{D}* D* close to threshold increases the cross section significantly and can not be neglected in comparison to the total dissociation rate. This is due to resonant bar{D}*-D* interactions in sqrt{s} close to the masses of the Psi(4.04) and Psi(4.16) mesons. We also investigate thermal effects of the (c bar{c}) width for such processes in the medium. All these effects change both the magnitude and the shape of the cross section as a function of sqrt{s}. The results obtained should be applied in the analysis of open and hidden charm production in heavy-ion collisions.

hep-ph↗

arXiv2005

Studying unquenching effects in QCD with Dyson-Schwinger equations

We summarise recent results on the properties of gluons, quarks and light mesons from the Green's functions approach to QCD. We discuss a self-consistent, infrared power law solution for the Schwinger-Dyson equations of the 1PI-Greens functions of Yang-Mills theory. The corresponding running coupling has a universal fixed point at zero momentum. Based on these analytical results a truncation scheme for the coupled system of Schwinger-Dyson equations for the propagators of QCD and the Bethe-Salpeter equation for light mesons has been formulated. We compare numerical results for charge eigenstate vector and pseudoscalar meson observables with corresponding lattice data. The effects of unquenching the system are found to be small but not negligible.

hep-ph↗

arXiv2004

Unquenching the Quark-Antiquark Green's Function

We propose a nonperturbative resummation scheme for the four-point connected quark-antiquark Green's function $G^4$ that shows how the Bethe-Salpeter equation may be `unquenched' with respect to quark-antiquark loops. This mechanism allows to dynamically account for hadronic meson decays and multiquark structures whilst respecting the underlying symmetries. An initial approximation to the four-point Schwinger-Dyson equation -- suitable for phenomenological application -- is examined numerically in a couple of aspects. It is demonstrated that this approximation explicitly maintains the correct asymptotic limits and contains the physical resonance structures in the near timelike region in the quark-antiquark channel whereas no resonances are found in the diquark channel, respectively.

hep-ph↗

arXiv1995

Relativistic Transport Approach for Nucleus-Nucleus Collisions from SIS to SPS Energies

We formulate a covariant transport approach for high energy nucleus-nucleus collisions where the real part of the nucleon selfenergies is fitted to nuclear matter properties which are evaluated on the basis of a NJL-type Lagrangian for the quark degrees of freedom. The parameters of the quark-model Lagrangian are fixed by the Gell-Mann, Oakes and Renner relation, the pion- nucleon $Σ$-term, the nucleon energy as well as the nuclear binding energy at saturation density $ρ_0$. We find the resulting scalar and vector selfenergies for nucleons to be well in line with either Dirac-Brueckner computations for $ρ\leq 2 ρ_0$ or those from the phenomenological optical potential when accounting for a swelling of the nucleon at finite nuclear matter density. The meson-baryon interaction density is modelled to describe a decrease of the meson mass with baryon density. The imaginary part of the hadron selfenergies is determined by a string fragmentation model which accounts for the in-medium mass of hadrons in line with the 'chiral' dynamics employed. The applicability of the transport approach is demonstrated in comparison with experimental data from SIS to SPS energies. The enhancement of the K$^+/π^+$ ratio in A + A collisions compared to p + A reactions at AGS energies is reproduced within the 'chiral' dynamics. Furthermore, detailed predictions for the stopping in Pb + Pb collisions at 153 GeV/A are presented.

hep-ph↗

arXiv1995

Cluster Expansion Approach to the Effective Potential in $Φ^4_{2+1}$-Theory

We apply a truncated set of dynamical equations of motion for connected equal-time Green functions up to the 4-point level to the investigation of spontaneous ground state symmetry breaking in $Φ^4_{2+1}$ quantum field theory. Within our momentum space discretization we obtain a second order phase transition as soon as the connected 3-point function is included. However, an additional inclusion of the connected 4-point function still shows a significant influence on the shape of the effective potential and the critical coupling.

hep-th↗

arXiv1994

Vibrations versus collisions and the iterative structure of two-body dynamics

We adopt a truncated version of two-body dynamics by neglecting three-body correlations, as is supported by microscopic numerical calculations. Introducing orthogonal channel correlations for the pp- and the ph-channel and integrating the latter in terms of vibrational RPA-states we derive a retarded two-body equation. Its solution is nonperturbative with respect to loops, ladders and mixed contributions. In the stationary limit we obtain an equation for a generalised effective interaction which iterates both the G-matrix and the polarisation matrix. An in-medium scattering approach transparently demonstrates the collisional damping of the vibrations.

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arXiv1994

Correlation Dynamics of Green's Functions

We generalize the methods used in the theory of correlation dynamics and establish a set of equations of motion for many-body correlation green's functions in the non-relativistic case. These non-linear and coupled equations of motion describe the dynamical evolution of correlation green's functions of different order and transparently show how many-body correlations are generated by the different interaction terms in a genuine nonperturbative framework. The nonperturbative results of the conventional green's function theory are included in the present formalism as two limiting cases (the so-called ladder diagram summation and ring diagram summation) as well as the familiar correlation dynamics of density-matrices in the equal time limit. We present explicit expressions for three- and four-body correlation functions that can be used to dynamically restore the trace relations for spin-symmetric fermi systems and study numerically the relative importance of two-, three- and four-body correlations for nuclear configurations close to the groundstate.

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arXiv1994

Analysis of Subthreshold Antiproton Production in p-Nucleus and Nucleus-Nucleus Collisions in the RBUU Approach

We calculate the subthreshold production of antiprotons in the Lorentz-covariant RBUU approach employing a weighted testparticle method to treat the antiproton propagation and absorption nonperturbatively. We find that the pbar differential cross sections are highly sensitive to the baryon and antiproton selfenergies in the dense baryonic environment. Adopting the baryon scalar and vector selfenergies from the empirical optical potential for proton-nucleus elastic scattering and from Dirac-Brueckner calculations at higher density rho > rho_0 we examine the differential pbar spectra as a function of the antiproton selfenergy. A detailed comparison with the available experimental data for p-nucleus and nucleus-nucleus reactions shows that the antiproton feels a moderately attractive mean-field at normal nuclear matter density rho_0 which is in line with a dispersive potential extracted from the free annihilation cross section.

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arXiv1993

Microscopic analysis of two-body correlations in light nuclei

Within a nonperturbative dynamical two-body approach - based on coupled equations of motion for the one-body density matrix and the two-body correlation function - we study the distribution of occupation numbers in a correlated system close to the groundstate, the relaxation of single-particle excitations and the damping of collective modes. For this purpose the nonlinear equations of motion are solved numerically within a finite oscillator basis for the first time adopting short-range repulsive and long-range attractive two-body forces. We find in all cases that the formation of long- and short-range correlations and their mixing is related to the long- and short-range part of the nucleon-nucleon interaction which dominate the resummation of loop or ladder diagrams, respectively. However, the proper description of relaxation or damping phenomena is found to require both types of diagrams as well as the mixed terms simultaneously.

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arXiv1993

Study of High Energy Heavy-Ion Collisions in a Relativistic BUU-Approach with Momentum-Dependent Mean-Fields

We introduce momentum-dependent scalar and vector fields into the Lorentz covariant relativistic BUU- (RBUU-) approach employing a polynomial ansatz for the relativistic nucleon-nucleon interaction. The momentum-dependent parametrizations are shown to be valid up to about 1 GeV/u for the empirical proton-nucleus optical potential. We perform numerical simulations for heavy-ion collisions within the RBUU-approach adopting momentum-dependent and momentum-independent mean-fields and calculate the transverse flow in and perpendicular to the reaction plane, the directivity distribution as well as subthreshold K+-production. By means of these observables we discuss the particular role of the momentum-dependent forces and their implications on the nuclear equation of state. We find that only a momentum-dependent parameter-set can explain the experimental data on the transverse flow in the reaction plane from 150 - 1000 MeV/u and the differential K+-production cross sections at 1 GeV/u at the same time.

nucl-th↗

arXiv1993

Antiproton Production in p-Nucleus and Nucleus-Nucleus Collisions within a Relativistic Transport Approach

The production of antiprotons in proton-nucleus and nucleus-nucleus reactions is calculated within the relativistic BUU approach employing proper selfenergies for the baryons and antiprotons and treating the p-bar annihilation nonperturbatively. The differential cross section for the antiprotons is found to be very sensitive to the p-bar selfenergy adopted. A detailed comparison with the available experimental data for p-nucleus and nucleus-nucleus reactions shows that the antiproton feels a moderately attractive mean-field at normal nuclear matter density which is in line with a dispersive potential extracted from the free annihilation cross section.

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