arXiv subjects

Jan Steinheimer

Publications and source records attributed to Jan Steinheimer.

123 records · Page 7Linked to original sources

arXiv2013

Elliptic flow and nuclear modification factors of D-mesons at FAIR in a Hybrid-Langevin approach

The Compressed Baryonic Matter (CBM) experiment at the Facility for Anti-proton and Ion Research (FAIR) will provide new possibilities for charm-quark ($D$-meson) observables in heavy-ion collisions at low collision energies and high baryon densities. To predict the collective flow and nuclear modification factors of charm quarks in this environment, 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 hydrodynamical evolution, the UrQMD hybrid approach provides a realistic evolution of the matter produced in heavy-ion collisions. 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 $130\, \MeV$. Hadronization of the charm quarks to $D$-mesons by coalescence is included. Since the initial charm-quark distribution at FAIR is unknown, we utilize two different initial charm-quark distributions in our approach to estimate the uncertainty of these predictions. We present calculations of the nuclear modification factor, $R_{AA}$, as well as for the elliptic flow, $v_2$, in Pb+Pb collisions at $E_{lab}=25\,\text{AGeV}$. The different medium modifications of $D$-mesons and $\bar{D}$-mesons at high baryon-chemical potential are explored by modified drag- and diffusion-coefficients using the corresponding fugacity factor. Here we find a considerably larger medium modification for $\bar{\mathrm{D}}$- than for $D$-mesons.

hep-ph↗

arXiv2013

Non-thermal p/pi ratio at LHC as a consequence of hadronic final state interactions

Recent LHC data on Pb+Pb reactions at sqrt(s_{NN})=2.7 TeV suggests that the p/pi is incompatible with thermal models. We explore several hadron ratios (K/pi, p/pi, Lambda/pi, Xi/pi) within a hydrodynamic model with hadronic after burner, namely UrQMD 3.3, and show that the deviations can be understood as a final state effect. We propose the p/pi as an observable sensitive on whether final state interactions take place or not. The measured values of the hadron ratios do then allow to gauge the transition energy density from hydrodynamics to the Boltzmann description. We find that the data can be explained with transition energy densities of 840 +- 150 MeV/fm^3.

nucl-th↗

arXiv2012

Heavy quark transport at RHIC and LHC

We calculate the heavy quark evolution in heavy ion collisions and show results for the elliptic flow $v_2$ as well as the nuclear modification factor $R_{AA}$ at RHIC and LHC energies. For the calculation we implement a Langevin approach for the transport of heavy quarks in the UrQMD (hydrodynamics + Boltzmann) hybrid model. 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 $130\,\text{MeV}$. At RHIC energies we use a coalescence approach at the decoupling temperature for the hadronization of the heavy quarks to D-mesons and B-mesons and a sub-following decay to heavy flavor electrons using PYTHIA. At LHC we use an additional fragmentation mechanism to account for the higher transverse momenta reached at higher collision energies.

hep-ph↗

arXiv2012

Heavy quark transport in heavy ion collisions at RHIC and LHC within the UrQMD transport model

We have implemented a Langevin approach for the transport of heavy quarks in the UrQMD hybrid model. The UrQMD hybrid approach provides a realistic description of the background medium for the evolution of relativistic heavy ion collisions. We have used two different sets of drag and diffusion coefficients, one based on a $T$-Matrix approach and one based on a resonance model for the elastic scattering of heavy quarks within the medium. In case of the resonance model we have investigated the effects of different decoupling temperatures of the heavy quarks from the medium, ranging between $130\,\text{MeV}$ and $180\,\text{MeV}$. We present calculations of the nuclear modification factor $R_{AA}$, as well as of the elliptic flow $v_2$ in Au+Au collisions at $\sqrt{s_{NN}}=200\,\text{GeV}$ and Pb+Pb collisions at $\sqrt{s_{NN}}=2.76\,\text{TeV}$. To make our results comparable to experimental data at RHIC and LHC we have implemented a Peterson fragmentation and a quark coalescence approach followed by the semileptonic decay of the D- and B-mesons to electrons. We find that our results strongly depend on the decoupling temperature and the hadronization mechanism. At a decoupling temperature of $130\,\text{MeV}$ we reach a good agreement with the measurements at both, RHIC and LHC energies, simultaneously for the elliptic flow $v_2$ and the nuclear modification factor $R_{AA}$.

hep-ph↗

arXiv2012

Spinodal amplification of density fluctuations in fluid-dynamical simulations of relativistic nuclear collisions

Extending a previously developed two-phase equation of state, we simulate head-on relativistic lead-lead collisions with fluid dynamics, augmented with a finite-range term, and study the effects of the phase structure on the evolution of the baryon density. For collision energies that bring the bulk of the system into the mechanically unstable spinodal region of the phase diagram, the density irregularities are being amplified significantly. The resulting density clumping may be exploited as a signal of the phase transition, possibly through an enhanced production of composite particles.

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arXiv2012

Charm quark transport in Pb+Pb reactions at sqrt{s}_NN=2.76 TeV from a (3+1) dimensional hybrid approach

We implement 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, this 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. 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 MeV. A coalescence approach at the decoupling temperature for the hadronization of the charm quarks to D-mesons is also included. We present calculations of the nuclear modification factor R_AA as well as the elliptic flow v_2 in Pb+Pb collisions at sqrt s_NN=2.76 TeV. The comparison to ALICE measurements shows a very good agreement with our calculations.

hep-ph↗

arXiv2012

Extraction of the sound velocity from rapidity spectra: Evidence for QGP formation at FAIR/RHIC-BES energies

We analyze longitudinal pion spectra from \sqrt{s_{NN}}= 2 GeV to \sqrt {s_{\rm NN}}=20 GeV within Landau's hydrodynamical model and the UrQMD hybrid approach. From the measured data on the widths of the pion rapidity spectra, we extract the sound velocity c_s^2 in the dense stage of the reactions. It is found that the sound velocity has a local minimum (indicating a softest point in the equation of state, EoS) at \sqrt{s_{NN}}= 4-9 GeV, an energy range accessible at the Facility for Antiproton and Ion Research (FAIR) as well as the RHIC-Beam Energy Scan (RHIC-BES). This softening of the EoS is compatible with the formation of a QGP at the onset of deconfinement. The extracted sound velocities are then used to calculate an excitation function for the mean transverse mass of pions from the hybrid model. We find that, above \sqrt{s_{NN}} \approx 10 GeV, even the lowest c_s^{2} gives a considerably larger of pions compared to data.

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arXiv2012

Baryon Resonances in a Chiral Hadronic Model for the QCD Equation of State

In this paper we study the influence of hadronic resonances on the phase diagram calculated with an effective chiral flavour SU(3) model. We show that varying the couplings of the baryonic resonances to the attractive scalar and the repulsive vector fields has a major impact on the order and location of the chiral phase transition and the possible existence of a critical end point as well as on the thermodynamic properties of the model. Furthermore, we study (strange) quark number fluctuations and show the related susceptibilities both at zero baryochemical potential and when crossing the phase transition line at three different points in the T-mu plane. We obtain the best agreement with current lattice data if we choose a rather strong vector coupling which in our model limits the phase transition to a smooth crossover and implies the non-existence of a critical end point.

hep-ph↗

arXiv2010

Conical Emission from Shock Waves in Ne(1-20 AGeV)+U Collisions

The formation and propagation of high-density compression waves, e.g. Mach shock waves, in cold nuclear matter is studied by simulating high-energy nucleus-nucleus collisions of Ne with U in the energy range from E_lab = 0.5 AGeV to 20 AGeV. In an ideal hydrodynamic approach, the high-density shock wave created by the small Ne nucleus passing through the heavy U nucleus is followed by a slower and more dilute Mach shock wave which causes conical emission of particles at the Mach cone angle. The conical emission originates from low-density regions with a small flow velocity comparable to the speed of sound. Moreover, it is shown that the angular distributions of emitted baryons clearly distinguish between a hydrodynamic approach and binary cascade processes used in the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) transport model.

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arXiv2010

Strangeness production in hadronic models and recombination models

We present recent results on the production, spectra and elliptic flow of strange particles in dynamic simulations employing hadronic degrees of freedom and from recombination models. The main focus will be on the Ultra-relativistic Molecular Dynamics (UrQMD) Boltzmann approach to relativistic heavy ion collisions and a hybrid approach with intermediate hydrodynamic evolution based on UrQMD (available for download as UrQMD v3.3). Compared to the standard binary collision approach, an enhancement of the strange particle particle yields is found in the hybrid approach due to the assumption of local equilibration. The production origins of the Phi-meson in the hybrid approach are studied in further detail. We also present results on the transverse momentum spectra of baryon to meson ratios of strange particles. Due to the approximate energy independent scaling of this ratio as a function of p_T we argue, that a maximum in these spectra may not be a unique sign for quark coalescence but can be understood in terms of flow and fragmentation.

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arXiv2009

Strangeness fluctuations and MEMO production at FAIR

We apply a coupled transport-hydrodynamics model to discuss the production of multi-strange meta-stable objects in Pb+Pb reactions at the FAIR facility. In addition to making predictions for yields of these particles we are able to calculate particle dependent rapidity and momentum distributions. We argue that the FAIR energy regime is the optimal place to search for multi-strange baryonic object (due to the high baryon density, favouring a distillation of strangeness). Additionally, we show results for strangeness and baryon density fluctuations. Using the UrQMD model we calculate the strangeness seperation in phase space which might lead to an enhanced production of MEMOs compared to models that assume global thermalization.

hep-ph↗

arXiv2009

$ $ excitation function: Freeze-out and equation of state dependence

An integrated Boltzmann+hydrodynamics transport approach is applied to investigate the dependence of the mean transverse mass on the freeze-out and the equation of state over the energy range from $E_{\rm lab}=2-160A $GeV. This transport approach based on the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) with an intermediate hydrodynamic stage allows for a systematic comparison without adjusting parameters. We find that the multiplicities of pions and protons are rather insensitive to different freeze-out prescriptions and changes in the equation of state, but the yields are slightly reduced in the hybrid model calculation compared to a pure transport calculation while the (anti)kaon multiplicities are increased. The mean transverse mass excitation functions of all three particle species are found to be sensitive to the different freeze-out treatments as well as to the equation of state. We find that the bag model equation of state with a strong first order phase transition is in qualitative agreement with the experimentally observed step-like behaviour in the < m_T > excitation function. The hybrid model with a hadron gas equation of state leads to a overestimation of the < m_T >, especially at higher energies. However, non-equilibrium effects seem also to be substantial as is suggested by the comparison with standard UrQMD results.

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arXiv2009

Effects of a phase transition on HBT correlations in an integrated Boltzmann+Hydrodynamics approach

A systematic study of HBT radii of pions, produced in heavy ion collisions in the intermediate energy regime (SPS), from an integrated (3+1)d Boltzmann+hydrodynamics approach is presented. The calculations in this hybrid approach, incorporating an hydrodynamic stage into the Ultra-relativistic Quantum Molecular Dynamics transport model, allow for a comparison of different equations of state retaining the same initial conditions and final freeze-out. The results are also compared to the pure cascade transport model calculations in the context of the available data. Furthermore, the effect of different treatments of the hydrodynamic freeze-out procedure on the HBT radii are investigated. It is found that the HBT radii are essentially insensitive to the details of the freeze-out prescription as long as the final hadronic interactions in the cascade are taken into account. The HBT radii $R_L$ and $R_O$ and the $R_O/R_S$ ratio are sensitive to the EoS that is employed during the hydrodynamic evolution. We conclude that the increased lifetime in case of a phase transition to a QGP (via a Bag Model equation of state) is not supported by the available data.

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arXiv2008

Fully integrated transport approach to heavy ion reactions with an intermediate hydrodynamic stage

We present a coupled Boltzmann and hydrodynamics approach to relativistic heavy ion reactions. This hybrid approach is based on the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) transport approach with an intermediate hydrodynamical evolution for the hot and dense stage of the collision. Event-by-event fluctuations are directly taken into account via the non-equilibrium initial conditions generated by the initial collisions and string fragmentations in the microscopic UrQMD model. After a (3+1)-dimensional ideal hydrodynamic evolution, the hydrodynamical fields are mapped to hadrons via the Cooper-Frye equation and the subsequent hadronic cascade calculation within UrQMD proceeds to incorporate the important final state effects for a realistic freeze-out. This implementation allows to compare pure microscopic transport calculations with hydrodynamic calculations using exactly the same initial conditions and freeze-out procedure. The effects of the change in the underlying dynamics - ideal fluid dynamics vs. non-equilibrium transport theory - will be explored. The freeze-out and initial state parameter dependences are investigated for different observables. Furthermore, the time evolution of the baryon density and particle yields are discussed. We find that the final pion and proton multiplicities are lower in the hybrid model calculation due to the isentropic hydrodynamic expansion while the yields for strange particles are enhanced due to the local equilibrium in the hydrodynamic evolution. The results of the different calculations for the mean transverse mass excitation function, rapidity and transverse mass spectra for different particle species at three different beam energies are discussed in the context of the available data.

nucl-th↗

arXiv2008

A fully integrated Boltzmann+hydrodynamics approach: Multiplicities, transverse dynamics and HBT

hep-ph↗