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Horst Stoecker

Publications and source records attributed to Horst Stoecker.

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

Analysis of hadron yield data within hadron resonance gas model with multi-component eigenvolume corrections

We analyze the sensitivity of thermal fits to heavy-ion hadron yield data of ALICE and NA49 collaborations to the systematic uncertainties in the hadron resonance gas (HRG) model related to the modeling of the eigenvolume interactions. We find a surprisingly large sensitivity in extraction of chemical freeze-out parameters to the assumptions regarding eigenvolumes of different hadrons. We additionally study the effect of including yields of light nuclei into the thermal fits to LHC data and find even larger sensitivity to the modeling of their eigenvolumes. The inclusion of light nuclei yields, thus, may lead to further destabilization of thermal fits. Our results show that modeling of eigenvolume interactions plays a crucial role in thermodynamics of HRG and that conclusions based on a non-interacting HRG are not unique.

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arXiv2016

Examination of directed flow as a signature of the softest point of the equation of state in QCD matter

We analyze the directed flow of protons and pions in high-energy heavy-ion collisions in the incident energy range from $\sqrt{s_{\scriptscriptstyle NN}}=7.7$ to 27 GeV within a microscopic transport model. Standard hadronic transport approaches do not describe the collapse of directed flow below $\sqrt{s_{\scriptscriptstyle NN}}\simeq 20$ GeV. By contrast, a model which simulates effects of a softening of the equation of state, well describes the behavior of directed flow data recently obtained by the STAR Collaboration~\cite{STARv1}. We give a detailed analysis of how directed flow is generated. Particularly, we found that softening of effective equation of state at the overlapping region of two nuclei, i.e. the reaction stages where the system reaches high baryon density state, is needed to explain the observed collapse of proton directed flow within a hadronic transport approach.

hep-ph↗
arXiv2016

Gauge Theory by canonical Transformations

Electromagnetism, the strong and the weak interaction are commonly formulated as gauge theories in a Lagrangian description. In this paper we present an alternative formal derivation of U(1)-gauge theory in a manifestly covariant Hamilton formalism. We make use of canonical transformations as our guiding tool to formalize the gauging procedure. The introduction of the gauge field, its transformation behaviour and a dynamical gauge field Lagrangian/Hamiltonian are unavoidable consequences of this formalism, whereas the form of the free gauge Lagrangian/Hamiltonian depends on the selection of the gauge dependence of the canonically conjugate gauge fields.

hep-th↗
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.

hep-ph↗
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

Perturbative QCD Calculations of Elliptic Flow and Shear Viscosity in Au+Au Collisions at $\sqrt{s_{NN}}=200$ GeV

The elliptic flow $v_2$ and the ratio of the shear viscosity over the entropy density, $η/s$, of gluon matter are calculated from the perturbative QCD (pQCD) based parton cascade Boltzmann approach of multiparton scatterings. For Au+Au collisions at $\sqrt{s}=200$A GeV the gluon plasma generates large $v_2$ values measured at the BNL Relativistic Heavy Ion Collider. Standard pQCD yields $η/s\approx 0.08-0.15$ as small as the lower bound found from the anti-de Sitter/conformal field theory conjecture.

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arXiv2008

Exclusion of black hole disaster scenarios at the LHC

The upcoming high energy experiments at the LHC are one of the most outstanding efforts for a better understanding of nature. It is associated with great hopes in the physics community. But there is also some fear in the public, that the conjectured production of mini black holes might lead to a dangerous chain reaction. In this paper we summarize the most straight forward arguments that are necessary to rule out such doomsday scenarios.

hep-ph↗
arXiv2008

QCD plasma thermalization, collective flow and extraction of shear viscosity

Fast thermalization and elliptic flow of QCD matter found at the Relativistic Heavy Ion Collider (RHIC) are understood as the consequence of perturbative QCD (pQCD) interactions within a (3+1) dimensional parton cascade. The main contributions stem from pQCD-inspired bremsstrahlung. We extract the shear viscosity to entropy ratio, which is between 0.08 and 0.15.

hep-ph↗
arXiv2008

Susceptibilities and fluctuations in a Quark-Hadron System with Dynamical Recombination

Within the framework of the dynamical recombination approach implemented in the partonic/hadronic quark Molecular Dynamics (qMD) model, we investigate numerous QGP signals constructed from the correlations and fluctuations of conserved charges, namely charged particle ratio fluctuations, charge transfer fluctuations as well as various ratios of susceptibilities. We argue that more generally, the covariances and the variances of the conserved charges divided by the number of charged particles are a measure of the quark number susceptibilities and are thus sensitive to the phase of the system. Computations carried within samples of central qMD events from low AGS energies on ($E_{lab}=2$ AGeV) up to the highest RHIC energies available ($\sqrt{s_{NN}}=200$ GeV) show that the final state calculations are always compatible with the hadronic result. From analyses performed as a function of time with our sample of events at $\sqrt{s_{NN}}=200$ GeV, we find the recombination-like hadronization process the qMD system undergoes to be responsible for the vanishing of the early stage QGP correlations and fluctuations. These results might explain the compatibility of the measurements on charged particle ratio fluctuations with the hadronic expectations and leave no room for the experimental observation of any QGP fluctuation/correlation signals based upon the susceptibilities of the conserved charges.

hep-ph↗
arXiv2008

Transport model study of the $m_T$-scaling for $Λ$, K, and $π$ HBT-correlations

Based on the microscopic transport model UrQMD in which hadronic and string degrees of freedom are employed, the HBT parameters in the longitudinal co-moving system are investigated for charged pion and kaon, and $Λ$ sources in heavy ion collisions (HICs) at SPS and RHIC energies. In the Cascade mode, $R_O$ and the $R_L$ at high SPS and RHIC energies do not follow the $m_T$-scaling, however, after considering a soft equation of state with momentum dependence (SM-EoS) for formed baryons and a density-dependent Skyrme-like potential for ``pre-formed'' particles, the HBT radii of pions and kaons and even those of $Λ$s with large transverse momenta follow the $m_T$-scaling function $R=3/\sqrt{m_T}$ fairly well.

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arXiv2007

The effect of "pre-formed" hadron potentials on the dynamics of heavy ion collisions and the HBT puzzle

The nuclear stopping, the elliptic flow, and the HBT interferometry are calculated by the UrQMD transport model, in which potentials for "pre-formed" particles (string fragments) from color fluxtube fragmentation as well as for confined particles are considered. This description provides stronger pressure at the early stage and describes these observables better than the default cascade mode (where the "pre-formed" particles from string fragmentation are treated to be free-streaming). It should be stressed that the inclusion of potential interactions pushes down the calculated HBT radius $R_O$ and pulls up the $R_S$ so that the HBT time-related puzzle disappears throughout the energies from AGS, SPS, to RHIC.

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arXiv2007

MACE -- Mach cones in heavy ion collisions

We study the propagation of sound-like perturbations created by a jet moving with supersonic velocity through the quark-gluon-plasma created in heavy-ion reactions within the model MACE (MAch Cone Evolution). Predictions for heavy-ion reactions at RHIC energies (Au+Au-collisions) and for Pb+Pb reactions at the LHC ($\sqrt s=5.5A$ TeV) are presented and potential observations by the STAR, PHENIX and ALICE experiments are 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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arXiv2007

Pion freeze-out as seen through HBT correlations in heavy ion collisions from FAIR/AGS to RHIC energies

We perform a systematic analysis of several HBT parameters in heavy ion collisions from $E_{\rm beam}=2$ AGeV to $\sqrt {s_{\rm NN}}=200$ GeV within the UrQMD transport approach and compare the results to experimental data where available. We find that the 'lifetime' of the emission source as calculated from $τ\sim \sqrt{R_O^{2}-R_S^{2}}$, is larger than the experimentally observed values at all investigated energies. The calculated volume of the pion source ($V_f$) is found to increase monotonously with increasing beam energy and the experimentally observed decrease of the measured $V_f$ at AGS is not seen. Finally, we calculate the mean free path $λ_f=0.5-1$ fm of pions at freeze-out and find a good description of the experimental data above the AGS energy region, supporting the suggestion of a universal kinetic decoupling criterion up to the highest RHIC energies.

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