arXiv subjects

G. Burau

Publications and source records attributed to G. Burau.

At least 19 recordsLinked to original sources

arXiv2009

A transport calculation with an embedded (3+1)d hydrodynamic evolution: Elliptic flow as a function of transverse momentum at SPS energies

The transverse momentum dependence of elliptic flow of pions calculated in a full (3+1)d Boltzmann approach with an intermediate hydrodynamic stage for heavy ion reactions for CERN-SPS energies is discussed in the context of the experimental data. At higher SPS energies, where the pure transport calculation cannot account for the high elliptic flow values, the smaller mean free path in the hydrodynamic evolution leads to larger elliptic flow. Due to the more realistic initial conditions and the incorporated hadronic rescattering the results are in line with the experimental data. Within this integrated dynamical approach different equations of state are applied without adjusting the initial state and the freeze-out conditions. We employ a hadron gas equation of state to investigate the differences in the dynamics and viscosity effects, a chiral equation of state with a moderate first order phase transition and a critical endpoint and a bag model equation of state with a large latent heat. The elliptic flow results are shown to be rather insensitive to changes in the equation of state.

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arXiv2009

Equation of state at FAIR energies and the role of resonances

Two microscopic models, UrQMD and QGSM, are used to extract the effective equation of state (EOS) of locally equilibrated nuclear matter produced in heavy-ion collisions at energies from 11.6 AGeV to 160 AGeV. Analysis is performed for the fixed central cubic cell of volume V = 125 fm**3 and for the expanding cell that followed the growth of the central area with uniformly distributed energy. For all reactions the state of local equilibrium is nearly approached in both models after a certain relaxation period. The EOS has a simple linear dependence P/e = c_s**2 with 0.12 < c_s**2 < 0.145. Heavy resonances are shown to be responsible for deviations of the c_s**2(T) and c_s**2(mu_B) from linear behavior. In the T-mu_B and T-mu_S planes the EOS has also almost linear dependence and demonstrates kinks related not to the deconfinement phase transition but to inelastic freeze-out in the system.

hep-ph↗

arXiv2008

Parton rearrangement and fast thermalization in heavy-ion collisions at RHIC and LHC energies

The implications of parton rearrangement processes on the dynamics of ultra-relativistic heavy-ion collisions have been investigated. A microscopic transport approach, namely the quark gluon string model (QGSM) which has been extended for a locally density-dependent partonic rearrangement and fusion procedure served as the tool for this investigations. The model emulates effectively the dynamics of a strongly coupled quark plasma and final hadronic interactions. Main QGSM results on anisotropic flow components v1 and v2 at top RHIC energy are compiled. Predictions for the pseudorapidity dependence of directed and elliptic flow in Pb+Pb collisions under LHC conditions are presented.

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arXiv2008

Microscopic models and effective equation of state in nuclear collisions at FAIR energies

Two microscopic models, UrQMD and QGSM, were employed to study the formation of locally equilibrated hot and dense nuclear matter in heavy-ion collisions at energies from 11.6 AGeV to 160 AGeV. Analysis was performed for the fixed central cubic cell of volume V = 125 fm**3 and for the expanding cell which followed the growth of the central area with uniformly distributed energy. To decide whether or not the equilibrium was reached, results of the microscopic calculations were compared to that of the statistical thermal model. Both dynamical models indicate that the state of kinetic, thermal and chemical equilibrium is nearly approached at any bombarding energy after a certain relaxation period. The higher the energy, the shorter the relaxation time. Equation of state has a simple linear dependence P = a(sqrt{s})*e, where a = c_s**2 is the sound velocity squared. It varies from 0.12 \pm 0.01 at E_{lab} = 11.6 AGeV to 0.145 \pm 0.005 at E_{lab} = 160 AGeV. Change of the slope in a(sqrt{s}) behavior occurs at E_{lab} = 40 AGeV and can be assigned to the transition from baryon-rich to meson-dominated matter. The phase diagrams in the T - mu_B plane show the presence of kinks along the lines of constant entropy per baryon. These kinks are linked to the inelastic (i.e. chemical) freeze-out in the system.

hep-ph↗

arXiv2007

Anisotropic flow in Pb+Pb collisions at LHC from the quark gluon string model with parton rearrangement

We present predictions for the pseudorapidity dependence of the azimuthal anisotropy parameters v_1 and v_2 of baryons and inclusive charged hadrons in Pb+Pb collisions at a LHC energy of sqrt(s) = 5.5 TeV applying a microscopic transport model, namely the quark gluon string model (QGSM) which has been recently extended for parton rearrangement and fusion processes. Pb+Pb collisions with impact parameters b = 2.3 fm and b = 8 fm have been simulated in order to investigate additionally the difference between central and semiperipheral configurations. In contrast to v_1(eta) at RHIC, the directed flow of charged hadrons shows a small normal flow alignment. The elliptic flow v_2(eta) turns out to be rather similar in shape for RHIC and LHC conditions, the magnitude however increases about 10-20 % at the LHC, leading to the conclusion that the hydrodynamical limit will be reached.

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arXiv2007

Pseudorapidity shape of elliptic flow as signature for fast equilibration in relativistic heavy-ion collisions at energies up to sqrt(s) = 200 GeV

The implications of parton recombination processes on the dynamics of ultrarelativistic heavy-ion reactions are investigated. To do so, the quark-gluon string transport model has been extended for partonic recombination and fusion processes. Parton recombination leads to short equilibration times and improves significantly on the theoretical description of measured directed and elliptic flow, i.e., v_1 and v_2, distributions in Au+Au collisions at sqrt(s) = 200 GeV, in particular what concerns their pseudorapidity dependence. The shape of v_2(eta) is found to be closely related to fast thermalization.

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arXiv2006

Medium effects on phi decays to dilepton and kaon-antikaon pairs in relativistic heavy ion reactions

We consider the role of rescattering of secondary kaons on the dilepton branching ratio of the phi meson. In-medium mass modifications and broadening of kaons and phi mesons are taken into account. We find in the framework of a Bjorken scenario for the time evolution of the expanding fireball that the phi yield from dimuons is moderately or at least only slightly enhanced compared to that from kaon-antikaon pairs. The relation to experimental yields measured by the NA49, NA50 and CERES Collaborations at CERN SPS and the PHENIX Collaboration at RHIC is discussed.

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arXiv2005

Energy densities and equilibration in heavy ion collisions at sqrt(s) = 200 GeV with the quark-gluon string model

We study thermodynamic characteristica of ultra-relativistic Au+Au collisions at RHIC energy sqrt(s) = 200 GeV within the framework of a microscopic transport model, namely the quark-gluon string model (QGSM). The temporal evolution of the local energy density, transverse and longitudinal pressure and equilibration times are considered. In contrast to complete equilibration which is even in central reactions hardly achieved, pre-equilibrium stages with energy densities well above the critical energy density predicted by lattice QCD are established at short time scales. Corresponding energy density profiles at proper time tau = 1 fm/c compare well with hydrodynamical assumptions for initial energy density distributions.

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arXiv2005

Anisotropic flow of strange particles at RHIC

Space-time picture of the anisotropic flow evolution in Au+Au collisions at BNL RHIC is studied for strange hadrons within the microscopic quark-gluon string model. The directed flow of both mesons and hyperons demonstrates wiggle structure with the universal antiflow slope at |y| < 2 for minimum bias events. This effect increases as the reaction becomes more peripheral. The development of both components of the anisotropic flow is closely related to particle freeze-out. Hadrons are emitted continuously, and different hadronic species are decoupled from the system at different times. These hadrons contribute differently to the formation and evolution of the elliptic flow, which can be decomposed onto three components: (i) flow created by hadrons emitted from the surface at the onset of the collision; (ii) flow produced by jets; (iii) hydrodynamic flow. Due to these features, the general trend in elliptic flow formation is that the earlier mesons are frozen, the weaker their elliptic flow. In contrast, baryons frozen at the end of the system evolution have stronger v2.

hep-ph↗

arXiv2005

Anisotropic flow of charged and identified hadrons in the quark-gluon string model for Au+Au collisions at sqrt(s) = 200 GeV

The pseudorapidity behaviour of the azimuthal anisotropy parameters v_1 and v_2 of inclusive charged hadrons and their dependence on the centrality has been studied in Au+Au collisions at full RHIC energy of sqrt(s) = 200 GeV within the microscopic quark-gluon string model. The QGSM simulation results for the directed flow v_1 show antiflow alignment within the pseudorapidity range |eta| < 2 in a fair agreement with the experimental v_1(eta) data, but cannot reproduce the further development of the antiflow up to |eta| around 3.5. The eta dependence of the elliptic flow v_2 extracted from the simulations agrees well with the experimental data in the whole pseudorapidity range for different centrality classes. The centrality dependence of the integrated elliptic flow of charged hadrons in the QGSM almost coincides with the PHOBOS experimental distribution. The transverse momentum dependence of the elliptic flow of identified and inclusive charged hadrons is studied also. The model reproduces quantitatively the low p_T part of the distributions rather good, but underestimates the measured elliptic flow for transverse momenta p_T > 1 GeV/c. Qualitatively, however, the model is able to reproduce the saturation of the v_2(p_T) spectra with rising p_T as well as the crossing of the elliptic flow for mesons and baryons.

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arXiv2004

Elliptic flow at RHIC: where and when does it formed?

Evolution of the elliptic flow of hadrons in heavy-ion collisions at RHIC energies is studied within the microscopic quark-gluon string model. The elliptic flow is shown to have a multi-component structure caused by (i) rescattering and (ii) absorption processes in spatially asymmetric medium. Together with different freeze-out dynamics of mesons and baryons, these processes lead to the following trend in the flow formation: the later the mesons are frozen, the weaker their elliptic flow, whereas baryon fraction develops stronger elliptic flow during the late stages of the fireball evolution. Comparison with the PHOBOS data demonstrates the model ability to reproduce the v2(eta) signal in different centrality bins.

hep-ph↗

arXiv2002

Mott Effect and J/Psi Dissociation at the Quark-Hadron Phase Transition

We investigate the in-medium modification of pseudoscalar and vector mesons in a QCD motivated chiral quark model by solving the Dyson-Schwinger equations for quarks and mesons at finite temperature for a wide mass range of meson masses, from light (pi, rho) to open-charm (D, D*) states. At the chiral / deconfinement phase transition, the quark-antiquark bound states enter the continuum of unbound states and become broad resonances (the hadronic Mott effect). We calculate the in-medium cross sections for charmonium dissociation due to collisions with light hadrons in a chiral Lagrangian approach, and show that the D and D* meson spectral broadening lowers the threshold for charmonium dissociation by pi and rho meson impact. This leads to a step-like enhancement in the reaction rate. We suggest that this mechanism for enhanced charmonium dissociation may be the physical mechanism underlying the anomalous J/Psi suppression observed by NA50.

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arXiv2002

Hadronic Spectral Function and Charm Meson Production

At the chiral restoration/deconfinement transition, most hadrons undergo a Mott transition from being bound states in the confined phase to resonances in the deconfined phase. We investigate the consequences of this qualitative change in the hadron spectrum on final state interactions of charmonium in hot and dense matter, and show that the Mott effect for D-mesons leads to a critical enhancement of the J/Psi dissociation rate. Anomalous J/Psi suppression in the NA50 experiment is discussed as well as the role of the Mott effect for the heavy flavor kinetics in future experiments at the LHC. The status of our calculations of hadron-hadron cross sections using the quark interchange and chiral Lagrangian approaches is reviewed, and an Ansatz for a unification of these schemes is given.

hep-ph↗

arXiv2001

Heavy flavor kinetics at the hadronization transition

We investigate the in-medium modification of the charmonium breakup processes due to the Mott effect for light (pi, rho) and open-charm (D, D*) quark-antiquark bound states at the chiral/deconfinement phase transition. The Mott effect for the D-mesons effectively reduces the threshold for charmonium breakup cross sections, which is suggested as an explanation of the anomalous J/psi suppression phenomenon in the NA50 experiment. Further implications of finite-temperature mesonic correlations for the hadronization of heavy flavors in heavy-ion collisions are discussed.

hep-ph↗

arXiv2001

Chiral Lagrangian approach to the J/psi breakup cross section

We summarize the results of the SU(4) chiral meson Lagrangian approach to the cross section for J/psi breakup by pion and rho meson impact and suggest a new scheme for the introduction of formfactors for the meson-meson interaction. This scheme respects the fact that on the quark level of description the contact and the meson exchange diagrams are constructed by so-called box and triangle diagrams which contain a different number of vertex functions for the quark-meson coupling. A model calculation for Gaussian vertex functions is presented and the relative importance of contact and meson exchange processes is discussed. We evaluate the dependence of the breakup cross section on the masses of the final D-meson states which can be used for the study of in-medium effects on this quantity.

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arXiv2001

Chiral quark model with infrared cut-off for the description of meson properties in hot matter

A simple chiral quark model of the Nambu--Jona-Lasinio (NJL) type with a quark confinement mechanism is constructed for the description of the light meson sector of QCD at finite temperature. Unphysical quark production thresholds in the NJL model are excluded by an infrared cut-off in the momentum integration within quark loop diagrams. This chiral quark model satisfies the low energy theorems. Using the vacuum masses and decay widths of π- and ρ-mesons for fixing the model parameters, the properties of the σ- meson are derived. Within the Matsubara formalism, the model is systematically extended to finite temperatures where chiral symmetry restoration due to a dropping constituent quark mass entails a vanishing of the infrared cut-off (deconfinement) at the pion Mott temperature T_c=186 MeV. Besides of the pion mass and weak decay constant, the masses, coupling constants and decay widths of σ- and ρ-mesons in hot matter are investigated. The quark-antiquark decay channel of the light mesons is opened for T>T_c only and becomes particularly strong for the ρ- meson. The two-pion decay channel below T_c has almost constant width for the ρ- meson up to T_c, but for the σ-meson it closes below T_c such that a scalar meson state with vanishing width is obtained as a precursor of the chiral/deconfinement transition.

hep-ph↗

arXiv2001

Finite T meson correlations and quark deconfinement

Finite temperature spatial $\bar q q$ correlation modes in the $π$ and $ρ$ channels are studied with the rainbow-ladder truncated quark Dyson-Schwinger equation and Bethe-Salpeter equation in the Matsubara formalism. To retain the finite range of the effective interaction while facilitating summation over fermion Matsubara modes necessary to ensure continuity at {T=0}, a separable kernel is used. The model is fixed by {T=0} properties and it implements dynamical chiral symmetry breaking and quark confinement. Above and below the deconfinement and chiral restoration transitions, we study $M_π(T)$, $f_π(T)$ and the 3-space transverse and longitudinal masses $M_ρ^T(T)$ and $M_ρ^L(T)$. For the $ρ$ mode we also study the strong and electromagnetic partial widths to determine the $T$-dependence that is generated by the quark content. The $ρ\to ππ$ width decreases sharply just above the chiral restoration transition leaving the $ρ$ with only its narrow {$e^+ e^-$} width. We discuss improvements needed by this model, especially in regard to the high $T$ behavior of the masses where we compare to lattice QCD simulations.

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arXiv2000

Mott dissociation of D-mesons at the chiral phase transition and anomalous J/Psi suppression

We investigate the in-medium modification of the charmonium breakup processes due to the Mott-effect for D-mesons at the chiral phase transition. A model calculation for the process J/Psi+pi --> D+D^*+ h.c. is presented which demonstrates that threshold effects in the thermal averaged breakup cross section can be explained as a Mott transition where final state quark-antiquark bound states enter the continuum of resonant states at the QCD phase transition. Applications to heavy-ion collisions within a modified Glauber model scenario and the phenomenon of anomalous J/$ψ$ suppression in the CERN NA50 experiment are addressed.

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