arXiv subjects

Elena Bratkovskaya

Publications and source records attributed to Elena Bratkovskaya.

At least 91 records · Page 5Linked to original sources

arXiv2017

The production of primordial $J/ψ$ in p+p and relativistic heavy-ion collisions

$J/ψ$ observables of p+p collisions at $\sqrt{s}=$ 200 GeV and 2.76 TeV are calculated using the PYTHIA event generator for the production of $c$ and $\bar c$ and the Wigner density formalism. The charm quark momentum from PYTHIA is tuned in such a way that the transverse momentum and rapidity distribution correspond to the Fixed-Order Next-to-Leading Logarithm (FONLL) calculations and hence to the experimental data for open charm mesons. Using the Wigner density of the charmonium wave function we calculate for each charm quark pair the probability to form a charmonium. As a result, the experimental data on the total $J/ψ$ yield, its rapidity and transverse momentum distribution as well as the fractions of feed-down from excited states of charmonium are reproduced. Applying the same approach to relativistic heavy-ion collisions, we find, if quark gluon plasma (QGP) effects are ignored, a considerable enhancement of the primordial $J/ψ$'s.

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arXiv2017

Evidence for chiral symmetry restoration in heavy-ion collisions

We study the effect of the chiral symmetry restoration (CSR) on heavy-ion collisions observables in the energy range $\sqrt{s_{NN}}$ = 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 particle production. Our systematic studies show that chiral symmetry restoration plays a crucial role in the description of heavy-ion collisions at $\sqrt{s_{NN}}$ = 3-20 GeV, realizing an increase of the hadronic particle production in the strangeness sector with respect to the non-strange one. Our results provide a microscopic explanation for the "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 $\sqrt{s_{NN}} \approx $ 7 GeV, while the drop at higher energies is associated to the appearance of a deconfined partonic medium. Furthermore, the appearance/disappearance of the "horn" structure is investigated as a function of the system size. We additionally present an analysis of strangeness production in the ($T,μ_B$)-plane (as extracted from the PHSD for central Au+Au collisions) and discuss the perspectives to identify a possible critical point in the phase diagram.

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arXiv2017

Traces of non-equilibrium dynamics in relativistic heavy-ion collisions

The impact of non-equilibrium effects on the dynamics of heavy-ion collisions is investigated by comparing a non-equilibrium transport approach, the Parton-Hadron-String-Dynamics (PHSD), to a 2D+1 viscous hydrodynamical model, which is based on the assumption of local equilibrium and conservation laws. Starting the hydrodynamical model from the same non-equilibrium initial condition as in the PHSD, using an equivalent lQCD Equation-of-State (EoS), the same transport coefficients, i.e. shear viscosity $η$ and the bulk viscosity $ζ$ in the hydrodynamical model, we compare the time evolution of the system in terms of energy density, Fourier transformed energy density, spatial and momentum eccentricities and ellipticity in order to quantify the traces of non-equilibrium phenomena. In addition, we also investigate the role of initial pre-equilibrium flow on the hydrodynamical evolution and demonstrate its importance for final state observables. We find that due to non-equilibrium effects, the event-by-event transport calculations show large fluctuations in the collective properties, while ensemble averaged observables are close to the hydrodynamical results.

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arXiv2016

$K^{*}$ vector meson resonances dynamics in heavy-ion collisions

We study the strange vector meson ($K^*, \bar K^*$) dynamics in relativistic heavy-ion collisions based on the microscopic Parton-Hadron-String Dynamics (PHSD) transport approach which incorporates partonic and hadronic degrees-of-freedom, a phase transition from hadronic to partonic matter - Quark-Gluon-Plasma (QGP) - and a dynamical hadronization of quarks and antiquarks as well as final hadronic interactions. We investigate the role of in-medium effects on the $K^*, \bar K^*$ meson dynamics by employing Breit-Wigner spectral functions for the $K^*$'s with self-energies obtained from a self-consistent coupled-channel G-matrix approach. Furthermore, we confront the PHSD calculations with experimental data for p+p, Cu+Cu and Au+Au collisions at energies up to $\sqrt{{s}_{NN}} = 200$~GeV. Our analysis shows that at relativistic energies most of the final $K^*$s (observed experimentally) are produced during the late hadronic phase, dominantly by the $K+ π\to K^*$ channel, such that the fraction of the $K^*$s from the QGP is small and can hardly be reconstructed from the final observables. The influence of the in-medium effects on the $K^*$ dynamics at RHIC energies is rather modest due to their dominant production at low baryon densities (but high meson densities), however, it increases with decreasing beam energy. Moreover, we find that the additional cut on the invariant mass region of the $K^*$ further influences the shape and the height of the final spectra. This imposes severe constraints on the interpretation of the experimental results.

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arXiv2016

A dynamical quasiparticle approach for the Quark-Gluon-Plasma bulk and transport properties

The properties of quantum-chromo dynamics (QCD) nowadays are accessable by lattice QCD calculations at vanishing quark chemical potential $μ_q$=0 but often lack a transparent physical interpretation. In this review we report about results from an extended dynamical quasiparticle model (DQPM$^*$) in which the effective parton propagators have a complex selfenergy that depends on the temperature $T$ of the medium as well as on the chemical potential $μ_q$ and the parton three-momentum ${\boldsymbol p}$ with respect to the medium at rest. It is demonstrated that this approach allows for a good description of QCD thermodynamics with respect to the entropy density, pressure etc. above the critical temperature $T_c \approx$ 158 MeV. Furthermore, the quark susceptibility $χ_q$ and the quark number density $n_q$ are found to be reproduced simultaneously at zero and finite quark chemical potential. The shear and bulk viscosities $η, ζ$, and the electric conductivity $σ_e$ from the DQPM$^*$ also turn out in close agreement with lattice results for $μ_q$ =0. The DQPM$^*$, furthermore, allows to evaluate the momentum $p$, $T$ and $μ_q$ dependencies of the partonic degrees of freedom also for larger $μ_q$ which are mandatory for transport studies of heavy-ion collisions in the regime 5 GeV $< \sqrt{s_{NN}} <$ 10 GeV. We finally calculate the charm quark diffusion coefficient $D_s$ -- evaluated from the differential cross sections of partons in the medium for light and heavy quarks by employing the propagators and couplings from the DQPM -- and compare to the available lattice data. It is argued that the complete set of observables allows for a transparent interpretation of the properties of hot QCD.

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arXiv2016

What are the early degrees of freedom in ultra-relativistic nucleus-nucleus collisions?

The Parton-Hadron-String-Dynamics (PHSD) transport model is used to study the impact on the choice of initial degrees of freedom on the final hadronic and electromagnetic observables in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. We find that a non-perturbative 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 $T_{μν}(x)$ just after the passage of the impinging nuclei. In case of the gluonic initial condition the formation of $s,{\bar s}$ pairs in the QGP proceeds rather slow such that the anti-strange quarks and accordingly the $K^+$ mesons do not achieve chemical equilibrium even in central Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. Accordingly, the $K^+$ rapidity distribution is suppressed in the gluonic scenario and in conflict with the data from the BRAHMS Collaboration. The proton and antiproton rapidity distributions also disfavor the 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 $\sqrt{s_{NN}}$ = 200 GeV due to a large contribution from other channels. It is argued that dilepton spectra in the invariant mass range 1.2 GeV $< M <$ 3 GeV will provide a definitive answer once the background from correlated $D$-meson decays is subtracted experimentally.

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arXiv2016

Hadron production within PHSD

We study the production of hadrons in nucleus-nucleus collisions 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. The essential impact of CSR is found in the Schwinger mechanism (for string decay) which fixes the ratio of strange to light quark production in the hadronic medium. Our studies provide a microscopic explanation for the maximum in the $K^+/π^+$ ratio at about 30 A GeV which only shows up if in addition to CSR a deconfinement transition to partonic degrees-of-freedom is incorporated in the reaction dynamics.

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arXiv2016

Charm production in Pb+Pb collisions at the Large Hadron Collider energy

We study charm production in Pb+Pb collisions at $\sqrt{s_{\rm NN}}=$2.76 TeV in the Parton-Hadron-String-Dynamics transport approach and the charm dynamics in the partonic and hadronic medium. The charm quarks are produced through initial binary nucleon-nucleon collisions by using the PYTHIA event generator taking into account the (anti-)shadowing incorporated in the EPS09 package. The produced charm quarks interact with off-shell massive partons in the quark-gluon plasma and are hadronized into $D$ mesons through coalescence or fragmentation close to the critical energy density, and then interact with hadrons in the final hadronic stage with scattering cross sections calculated in an effective Lagrangian approach with heavy-quark spin symmetry. The PHSD results show a reasonable $R_{\rm AA}$ and elliptic flow of $D$ mesons in comparison to the experimental data for Pb+Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV from the ALICE Collaboration. We also study the effect of temperature-dependent off-shell charm quarks in relativistic heavy-ion collisions. We find that the scattering cross sections are only moderately affected by off-shell charm degrees of freedom. However, the position of the peak of $R_{\rm AA}$ for $D$ mesons depends on the strength of the scalar partonic forces which also have an impact on the $D$ meson elliptic flow. The comparison with experimental data on the $R_{\rm AA}$ suggests that the repulsive force is weaker for off-shell charm quarks as compared to that for light quarks. Furthermore, the effects from radiative charm energy loss appear to be low compared to the collisional energy loss up to transverse momenta of $\sim$ 15 GeV/c.

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arXiv2015

The quark susceptibility in a generalized dynamical quasiparticle model

The quark susceptibility $χ_q$ 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-quasi-particle 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 $μ_q$. The shear viscosity $η$ and the electric conductivity $σ_e$ from the extended quasiparticle model (DQPM$^*$) also turn out in close agreement with lattice results for $μ_q$ =0. The DQPM$^*$, furthermore, allows to evaluate the momentum $p$, temperature $T$ and chemical potential $μ_q$ dependencies of the partonic degrees of freedom also for larger $μ_q$ which are mandatory for transport studies of heavy-ion collisions in the regime 5 GeV $< \sqrt{s_{NN}} <$ 10 GeV.

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arXiv2015

Strangeness At Extremes

We study the properties of strange mesons in vacuum and in the hot nuclear medium within unitarized coupled-channel effective theories. We determine transition probabilities, cross sections and scattering lengths for strange mesons. These scattering observables are of fundamental importance for understanding the dynamics of strangeness production and propagation in heavy-ion collisions.

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arXiv2015

(Anti-)strangeness production in heavy-ion collisions

The production and dynamics of strange and antistrange hadrons in heavy-ion reactions from $\sqrt{s_{NN}} \approx$ 3 GeV to 200 GeV is analyzed within the Parton-Hadron-String-Dynamics (PHSD) transport model. The PHSD results for strange baryon and antibaryon production are roughly consistent with the experimental data starting from upper SPS energies. Nevertheless, hadronic final state flavor-exchange reactions are important for the actual abundances, in particular at large rapidities where hadronic dynamics, parton fragmentation and string decay dominate. A striking disagreement between the PHSD results and the available data persists, however, for bombarding energies below $\sqrt{s_{NN}} \approx$ 8 GeV where the strangeness production is significantly underestimated as in earlier HSD studies. This finding implies that the strangeness enhancement seen experimentally at FAIR/NICA energies cannot be attributed to a deconfinement phase transition or crossover but probably involves the approximate restoration of chiral symmetry in the hadronic phase.

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arXiv2015

Observables in ultrarelativistic heavy-ion collisions from two different transport approaches for the same initial conditions

For nucleus-nucleus collisions at Relativistic-Heavy-Ion Collider (RHIC) energies we calculate observables in two conceptually transport theories, i.e. the n-body molecular dynamical model RSP and in the Parton-Hadron-String-Dynamics (PHSD), starting out from the same distribution of the initial energy density at the quark gluon plasma (QGP) formation time. The RSP is based on the Nambu Jona-Lasinio (NJL) Lagrangian whereas in PHSD the partons are described by the dynamical quasi-particle model (DQPM). Despite of the very different description of the parton properties, their interactions and of the hadronization in both approaches the final transverse momentum distributions of pions and kaons are rather similar. Thus it will be difficult to conclude from the final experimenal data on the details of the initial plasma properties.

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arXiv2015

Tomography of the Quark-Gluon-Plasma by Charm Quarks

We study charm production in ultra-relativistic heavy-ion collisions by using the Parton-Hadron-String Dynamics (PHSD) transport approach. The initial charm quarks are produced by the Pythia event generator tuned to fit the transverse momentum spectrum and rapidity distribution of charm quarks from Fixed-Order Next-to-Leading Logarithm (FONLL) calculations. The produced charm quarks scatter in the quark-gluon plasma (QGP) with the off-shell partons whose masses and widths are given by the Dynamical Quasi-Particle Model (DQPM) which reproduces the lattice QCD equation-of-state in thermal equilibrium. The relevant cross section are calculated in a consistent way by employing the effective propagators and couplings from the DQPM. Close to the critical energy density of the phase transition, the charm quarks are hadronized into $D$ mesons through coalescence and/or fragmentation depending on transverse momentum. The hadronized $D$ mesons then interact with the various hadrons in the hadronic phase with cross sections calculated in an effective lagrangian approach with heavy-quark spin symmetry. Finally, the nuclear modification factor $\rm R_{AA}$ and the elliptic flow $v_2$ of $D^0$ mesons from PHSD are compared with the experimental data from the STAR Collaboration for Au+Au collisions at $\sqrt{s_{\rm NN}}$ =200 GeV. We find that in the PHSD the energy loss of $D$ mesons at high $p_T$ can be dominantly attributed to partonic scattering while the actual shape of $\rm R_{AA}$ versus $p_T$ reflects the heavy quark hadronization scenario, i.e. coalescence versus fragmentation. Also the hadronic rescattering is important for the $\rm R_{AA}$ at low $p_T$ and enhances the $D$-meson elliptic flow $v_2$.

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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.

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arXiv2014

Anti-strange meson-baryon interaction in hot and dense nuclear matter

We present a study of in-medium cross sections and (off-shell) transition rates for the most relevant binary reactions for strange pseudoscalar meson production close to threshold in heavy-ion collisions at FAIR energies. Our results rely on a chiral unitary approach in coupled channels which incorporates the $s$- and $p$-waves of the kaon-nucleon interaction. The formalism, which is modified in the hot and dense medium to account for Pauli blocking effects, mean-field binding on baryons, and pion and kaon self-energies, has been improved to implement full unitarization and self-consistency for both the $s$- and $p$-wave interactions at finite temperature and density. This gives access to in-medium amplitudes in several elastic and inelastic coupled channels with strangeness content $S=-1$. The obtained total cross sections mostly reflect the fate of the $Λ(1405)$ resonance, which melts in the nuclear environment, whereas the off-shell transition probabilities are also sensitive to the in-medium properties of the hyperons excited in the $p$-wave amplitudes [$Λ$, $Σ$ and $Σ^*(1385)$]. The single-particle potentials of these hyperons at finite momentum, density and temperature are also discussed in connection with the pertinent scattering amplitudes. Our results are the basis for future implementations in microscopic transport approaches accounting for off-shell dynamics of strangeness production in nucleus-nucleus collisions.

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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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arXiv2014

Properties of strange vector mesons in dense and hot matter

We investigate the in-medium properties of strange vector mesons ($K^*$ and $\bar K^*$) in dense and hot nuclear matter based on chirally motivated models of the meson selfenergies. We parameterise medium effects as density or temperature dependent effective masses and widths, obtain the vector meson spectral functions within a Breit-Wigner prescription (as often used in transport simulations) and study whether such an approach can retain the essential features of full microscopic calculations. For $μ_B\ne 0$ the medium corrections arise from $\bar K^* (K^*) N$ scattering and the $\bar K^* (K^*) \to \bar K (K) π$ decay mode (accounting for in-medium $\bar K (K)$ dynamics). We calculate the scattering contribution to the $K^*$ selfenergy based on the hidden local symmetry formalism for vector meson nucleon interactions, whereas for the $\bar K^*$ selfenergy we implement recent results from a selfconsistent coupled-channel determination within the same approach. For $μ_B\simeq 0$ and finite temperature we rely on a phenomenological approach for the kaon selfenergy in a hot pionic medium consistent with chiral symmetry, and evaluate the $\bar K^* (K^*) \to \bar K (K) π$ decay width. The emergence of a mass shift at finite temperature is studied with a dispersion relation over the imaginary part of the vector meson selfenergy.

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arXiv2013

Strange and heavy mesons in hadronic matter

We present selected results on the properties of strange and heavy-flavoured mesons in a hot and dense nuclear medium, with emphasis in selfconsistent coupled-channel approaches based on the chiral Lagrangian. In the strangeness sector, we discuss how the enhanced reactivity of light strange vectors at FAIR conditions can be tied to in-medium effects on their predominant decay modes (e.g. $\bar K^*\to\bar K π$) and to the excitation of strange baryons in vector-meson nucleon interactions. In the heavy-flavour sector, we focus on recent determinations of the transport coefficients of charmed and bottomed mesons in a hadron gas at vanishing baryonic chemical potential. We comment on the role of microscopic transport simulations to establish a connection between theoretical models and experimental observables from heavy-ion collisions.

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