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Jan Steinheimer

Publications and source records attributed to Jan Steinheimer.

At least 109 records · Page 6Linked to original sources
arXiv2016

Equation of state dependence of directed flow in a microscopic transport model

We study the sensitivities of the directed flow in Au+Au collisions on the equation of state (EoS), employing the transport theoretical model JAM. The EoS is modified by introducing a new collision term in order to control the pressure of a system by appropriately selecting an azimuthal angle in two-body collisions according to a given EoS. It is shown that this approach is an efficient method to modify the EoS in a transport model. The beam energy dependence of the directed flow of protons is examined with two different EoS, a first-order phase transition and crossover. It is found that our approach yields quite similar results as hydrodynamical predictions on the beam energy dependence of the directed flow; Transport theory predicts a minimum in the excitation function of the slope of proton directed flow and does indeed yield negative directed flow, if the EoS with a first-order phase transition is employed. Our result strongly suggests that the highest sensitivity for the critical point can be seen in the beam energy range of $4.7\leq\srtNN\leq11.5$ GeV.

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arXiv2016

Formation of deuterons by coalescence: Consequences on the deuteron number fluctuations

Two scenarios for cluster production have since long been discussed in the literature: i) direct emission of the clusters from a (grand canonical) thermal source or ii) subsequent formation of the clusters by coalescence of single nucleons. While both approaches have been successfully applied in the past it has not yet been clarified which of the two mechanisms dominates the cluster production. We propose to use recently developed event-by-event techniques to study particle multiplicity fluctuations on nuclear clusters and employ this analysis to the deuteron number fluctuations to disentangle the two production mechanisms. We argue that for a grand canonical cluster formation, the cluster fluctuations will follow Poisson distribution, while for the coalescence scenario, the fluctuations will strongly deviate from the Poisson expectation. We estimate the effect to be 10% for the variance and up to a factor of 5 for the kurtosis of the deuteron number multiplicity distribution. Our prediction can be tested in the beam energy scan program at RHIC as well as experiments at the FAIR and NICA facilities.

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arXiv2016

Influence of kinematic cuts on the net charge distribution

The higher moments of the net charge distributions, e.g. the skewness and kurtosis, are studied within an infinite hadronic matter calculation in a transport approach. By dividing the box into several parts, the volume dependence of the fluctuations is investigated. After confirming that the initial distributions follow the expectations from a binomial distribution, the influence of quantum number conservation in this case the net charge in the system on the higher moments is evaluated. For this purpose, the composition of the hadron gas is adjusted and only pions and $ρ$ mesons are simulated to investigate the charge conservation effect. In addition, the effect of imposing kinematic cuts in momentum space is analysed. The role of resonance excitations and decays on the higher moments can also be studied within this model. This work is highly relevant to understand the experimental measurements of higher moments obtained in the RHIC beam energy scan and their comparison to lattice results and other theoretical calculations assuming infinite matter.

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arXiv2015

Sub-threshold $ϕ$ and $Ξ^-$ production by high mass resonances with UrQMD

We present a possible explanation for the deep sub-threshold, $ϕ$ and $Ξ^-$ production yields measured with the HADES experiment in Ar+KCl reactions at $E_{\mathrm{lab}}=1.76$ A GeV and present predictions for Au+Au reactions at $E_{\mathrm{lab}}=1.23$ A GeV. To explain the surprisingly high yields of $ϕ$ and $Ξ^-$ hadrons we propose new decay channels for high mass baryon resonances. These new decay channels are constrained by elementary $\mathrm{p+p}\rightarrow \mathrm{p+p+}ϕ$ cross sections, and $Ξ^-$ production in p+Nb. Based on the fits to the elementary reactions one obtains a satisfactorily description of $ϕ$ and $Ξ^-$ production in deep sub-threshold Ar+KCl reactions as well as the observed nuclear transparency ratio in proton induced $ϕ$ production in cold nuclear matter. The results implicate that no new medium effects are required to describe the rare strange particle production data in low energy nuclear collisions.

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arXiv2015

Net-proton number kurtosis and skewness in nuclear collisions: Influence of deuteron formation

We explore the influence of deuteron formation in the late stage of nucleus-nucleus reactions on the fluctuations observed in the final net-proton yields around midrapidity. At each investigated energy, the produced (anti-)proton yield at chemical freeze-out is assumed to fluctuate according to a Poisson distribution and in each event the probability for deuteron formation by coalescence is proportional to $(dN_{\mathrm{proton}}/dy)^2$. The protons that are then clustered in deuterons are usually not included in the experimental measurement of the net-proton fluctuations, therefore, we subtract these clustered protons from the final state proton number for the calculation of the net-proton fluctuations (the same is done in the anti-proton sector). Due to the non-linear deuteron formation probability the resulting distribution is not a Skellam distribution, but shows the interesting feature of a decrease in the kurtosis $κσ^2$ and a local maximum in the skewness $S σ$ observables as the collision energy decreases.

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arXiv2015

Glueballs amass at RHIC and LHC Colliders! - The early quarkless 1st order phase transition at $T=270$ MeV - from pure Yang-Mills glue plasma to GlueBall-Hagedorn states

The early stage of high multiplicity pp, pA and AA collider is represented by a nearly quarkless, hot, deconfined pure gluon plasma. According to pure Yang-Mills Lattice Gauge Theory, this hot pure glue matter undergoes, at a high temperature, $T_c = 270$ MeV, a first order phase transition into a confined Hagedorn-GlueBall fluid. These new scenario should be characterized by a suppression of high $p_T$ photons and dileptons, baryon suppression and enhanced strange meson production. We propose to observe this newly predicted class of events at LHC and RHIC.

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arXiv2015

Recent results from the UrQMD hybrid model for heavy ion collisions

These proceedings present recent results from transport-hydrodynamics-hybrid models for heavy ion collisions at relativistic energies. The main focus is on the absorption of (anti-)protons in the hadronic afterburner stage of the reaction, di-lepton production at SPS and heavy quark dynamics.

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arXiv2015

The beam energy dependence of collective flow in heavy ion collisions

The major goals of heavy ion research are to explore the phase diagram of quantum chromodynamics (QCD) and to investigate the properties of the quark gluon plasma (QGP), a new state of matter created at high temperatures and/or densities. Collective anisotropic flow is one of the most promising observables to gain insights about the properties of the system created in relativistic heavy ion reactions. The current status of the beam energy dependence of the first three Fourier coefficients of the azimuthal distribution of the produced particles $v_1$ to $v_3$ within hybrid transport plus hydrodynamics approaches are summarized.

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arXiv2014

What the collective flow excitation function can tell about the quark-gluon plasma

Recent STAR data from the RHIC beam energy scan (BES) show that the midrapidity slope dv1/dy of the directed flow v1 of net-protons changes sign twice within the collision energy range 7.7 - 39 GeV. To investigate this phenomenon, we study the collision energy dependence of v1 utilizing a Boltzmann + hydrodynamics hybrid model. Calculations with dynamically evolved initial and final state show no qualitative difference between an equation of state with a cross-over and one with a first-order phase transition, in contrast to earlier pure fluid predictions. Furthermore, our analysis of the elliptic flow v2 shows that pre-equilibrium transport dynamics are partially compensating for the diminished elliptic flow production in the hydrodynamical phase at lower energies, which leads to a qualitative agreement with STAR BES results in midcentral collisions. No compensation from transport is found in our model for integrated v3, which decreases from approx 0.02 at sqrt(sNN) = 27 GeV to approx 0.005 at sqrt(sNN) = 7.7 GeV in midcentral collisions.

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arXiv2014

Do lattice data constrain the vector interaction strength of QCD?

We show how repulsive interactions of deconfined quarks as well as confined hadrons have an influence on the baryon number susceptibilities and the curvature of the chiral pseudo critical line in effective models of QCD. We discuss implications and constraints for the vector interaction strength from comparisons to lattice QCD and comment on earlier constraints, extracted from the curvature of the transition line of QCD and compact star observables. Our results clearly point to a strong vector repulsion in the hadronic phase and near-zero repulsion in the deconfined phase.

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arXiv2013

Conserved Charge Fluctuations in a Chiral Hadronic Model including Hadrons and Quarks

In this work the susceptibility coefficients of the strange and non-strange quark number of second and fourth order are presented. The results at zero baryonchemical potential are obtained using a well tested chiral effective model including all known hadron degrees of freedom and additionally incorporating quarks and gluons in a PNJL model approach. Quark number susceptibilities are sensitive to the fundamental degrees of freedom in the model and signal the shift from massive hadrons to light quarks at the deconfinement transition by a sharp rise at the critical temperature. Furthermore, all susceptibilities are found to be largely suppressed by repulsive vector field interactions of the particles. In the hadronic sector vector repulsion of baryon resonances restrains fluctuations to a region determined in lattice QCD. However, in the quark sector above $T_c$ even small vector field interactions of quarks quench all fluctuations almost completely. For this reason, vector field interactions for quarks have to vanish in the deconfinement limit.

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arXiv2013

Phase Structure of QCD Matter in a Chiral Effective Model with Quarks

Using a unified hadron-quark effective model for the QCD equation of state, this paper studies the phase structure of strongly interacting matter in a wide range of temperature and baryonchemical potential. At small potentials the model yields a smooth cross-over to chirally restored matter with a transition temperature and curvature in line with recent lattice QCD estimates and thermal model fits of freeze-out curves. Trajectories of constant entropy per net baryon number show a clear dependence on the particle composition in the model and on repulsive vector field interactions. Although the model might feature a critical end-point at a rather high baryonchemical potential and low temperature, probing it in heavy-ion collisions might be highly challenging due to a large thermodynamic spread of matter in the collision fireball.

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arXiv2013

Hadron Formation in Relativistic Nuclear Collisions and the QCD Phase Diagram

We analyze hadrochemical freeze-out in central Pb+Pb collisions at CERN SPS and LHC energies. We determine the effects of baryon and antibaryon annihilation and/or regeneration occurring during the final cascade expansion stage of the collisions by deriving "survival factors" for each hadronic species and all energies considered by employing the UrQMD hybrid model. These survival factors are shown to resemble the pattern of data deviation from the statistical equilibrium calculations with the statistical model. We apply them in the SM data analysis, obtaining new points in the $T-μ_B$ plane which follow closely the parton-hadron phase boundary predicted by lattice QCD.

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arXiv2013

QCD Equation of State From a Chiral Hadronic Model Including Quark Degrees of Freedom

This work presents an effective model for strongly interacting matter and the QCD equation of state (EoS). The model includes both hadron and quark degrees of freedom and takes into account the transition of chiral symmetry restoration as well as the deconfinement phase transition. At low temperatures $T$ and baryonic densities $ρ_B$ a hadron resonance gas is described using a SU(3)-flavor sigma-omega model and a quark phase is introduced in analogy to PNJL models for higher $T$ and $ρ_B$. In this way, the correct asymptotic degrees of freedom are used in a wide range of $T$ and $ρ_B$. Here, results of this model concerning the chiral and deconfinement phase transitions and thermodynamic model properties are presented. Large hadron resonance multiplicities in the transition region emphasize the importance of heavy-mass resonance states in this region and their impact on the chiral transition behavior. The resulting phase diagram of QCD matter at small chemical potentials is in line with latest lattice QCD and thermal model results.

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arXiv2013

Correlated D-meson decays competing against thermal QGP dilepton radiation

The QGP that might be created in ultrarelativistic heavy-ion collisions is expected to radiate thermal dilepton radiation. However, this thermal dilepton radiation interferes with dileptons originating from hadron decays. In the invariant mass region between the $ϕ$ and $J/Ψ$ peak ($1\,$GeV$\lesssim M_{\ell^+ \ell^-} \lesssim 3 \,$GeV) the most substantial background of hadron decays originates from correlated D$\bar{\mathrm{D}}$-meson decays. We evaluate this background using a Langevin simulation for charm quarks. As background medium we utilize the well-tested UrQMD-hybrid model. The required drag and diffusion coefficients are taken from a resonance approach. The decoupling of the charm quarks from the hot medium is performed at a temperature of $130\,$MeV and as hadronization mechanism a coalescence approach is chosen. This model for charm quark interactions with the medium has already been successfully applied to the study of the medium modification and the elliptic flow at FAIR, RHIC and LHC energies. In this proceeding we present our results for the dilepton radiation from correlated D$\bar{\mathrm{D}}$ decays at RHIC energy in comparison to PHENIX measurements in the invariant mass range between 1 and 3 GeV using different interaction scenarios. These results can be utilized to estimate the thermal QGP radiation.

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arXiv2013

Dileptons from correlated D- and $\bar{\text{D}}$-meson decays in the invariant mass range of the QGP thermal radiation using the UrQMD hybrid model

Relativistic heavy-ion collisions produce a hot and dense thermalized medium, that is expected to emit thermal radiation in form of dileptons. These dileptons are not affected by the strong force and are therefore a clean probe for the possible creation of a Quark Gluon Plasma (QGP). However, electroweak decays of open-charm mesons are expected to induce a substantial background in the invariant mass region between the $ϕ$ and $J/Ψ$ peak ($1\,\text{GeV} \lesssim M_{\ell^+ \ell^-} \lesssim 3 \,\text{GeV}$) of the thermal QGP radiation. To evaluate this background radiation we apply a Langevin approach for the transport of charm quarks in the UrQMD (hydrodynamics + Boltzmann) hybrid model. Due to the inclusion of event-by-event fluctuations and a full (3+1)-dimensional hydrodynamic evolution, the UrQMD hybrid approach provides a more realistic model for the evolution of the matter produced in heavy ion collisions as compared to simple homogeneous fireball expansions usually employed before. As drag and diffusion coefficients we use a resonance approach for elastic heavy-quark scattering and assume a decoupling temperature of the charm quarks from the hot medium of $130\,\text{MeV}$. For the hadronization of the charm quarks we employ a coalescence approach at the decoupling temperature from the medium. In this letter we present our calculations of the D-meson correlations and the invariant mass spectra of the dilepton decays in heavy-ion collisions at FAIR, RHIC, and LHC energies using different interaction scenarios.

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arXiv2013

Spinodal density enhancements in simulations of relativistic nuclear collisions

We recently introduced a fluid-dynamical model for simulating relativistic nuclear collisions in the presence of a first-order phase transition and made explorative studies of head-on lead-lead collisions. We give here a more detailed account of this novel theoretical tool and carry out more exhaustive studies of the phase-separation dynamics. Extracting the density enhancement caused by the spinodal instabilities, the associated clump size distribution, and the resulting transverse flow velocity, we examine the sensitivity of these quantities to the strength of the gradient term that promotes the phase separation, to the details of the initial density fluctuations that form the seeds for the subsequent amplification, and to the equation of state.

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