arXiv subjects

Steffen A. Bass

Publications and source records attributed to Steffen A. Bass.

At least 73 records · Page 4Linked to original sources

arXiv2015

Charm Quark Energy Loss In Infinite QCD Matter Using A Parton Cascade Model

We utilize the Parton Cascade Model to study the evolution of charm quarks propagating through a thermal brick of QCD matter. We determine the energy loss and the transport coefficient '$\hat{q}$' for charm quarks. The calculations are done at a constant temperature of 350 MeV and the results are compared to analytical calculations of heavy quark energy loss in order to validate the applicability of using a Parton Cascade Model for the study of heavy quarks dynamics in hot and dense QCD matter.

nucl-th↗

arXiv2013

Heavy quark dynamics and hadronization in ultra-relativistic heavy-ion collisions: collisional versus radiative energy loss

We study the dynamics of energy loss and flow of heavy quarks produced in ultra-relativistic heavy-ion collisions within the framework of a Langevin equation coupled to a (2+1)-dimensional viscous hydrodynamic model that simulates the space-time evolution of the produced hot and dense QCD matter. The classical Langevin approach is improved such that, apart from quasi-elastic scatterings, radiative energy loss is incorporated by treating gluon radiation as an additional force term. The hadronization of emitted heavy quarks is simulated via a hybrid fragmentation plus recombination model. Our calculation shows significant contribution from gluon radiation to heavy quark energy loss at high energies, and we find the recombination mechanism is important for heavy flavor meson production at intermediate energies. We present numerical results for the nuclear modification and elliptic flow of D mesons, which are consistent with measurements at both LHC and RHIC; predictions for B mesons are also provided.

nucl-th↗

arXiv2013

Model and parameter dependence of heavy quark energy loss in a hot and dense medium

Within the framework of the Langevin equation, we study the energy loss of heavy quark due to quasi-elastic multiple scatterings in a quark-gluon plasma created by relativistic heavy-ion collisions. We investigate how the initial configuration of the quark-gluon plasma as well as its properties affect the final state spectra and elliptic flow of D meson and non-photonic electron. We find that both the geometric anisotropy of the initial quark-gluon plasma and the flow profiles of the hydrodynamic medium play important roles in the heavy quark energy loss process and the development of elliptic flow. The relative contribution from charm and bottom quarks is found to affect the transverse momentum dependence of the quenching and flow patterns of heavy flavor decay electron; such influence depends on the interaction strength between heavy quark and the medium.

nucl-th↗

arXiv2013

Center Domains and their Phenomenological Consequences

We argue that the domain structure of deconfined QCD matter, which can be inferred from the properties of the Polyakov loop, can simultaneously explain the two most prominent experimentally verified features of the quark-gluon plasma, namely its large opacity as well as its near ideal fluid properties.

nucl-th↗

arXiv2012

Collisional vs. Radiative Energy Loss of Heavy Quark in a Hot and Dense Nuclear Matter

We study the heavy quark evolution in a quark-gluon plasma medium within the framework of Langevin equation coupled to a (2+1)-dimensional viscous hydrodynamic model. We modify the current Langevin algorithm such that apart from quasi-elastic scattering, medium-induced radiative energy loss is incorporated as well by treating gluon radiation as an extra force term. We find a significant effect of gluon radiation on heavy quark energy loss at LHC energies. Our calculation provides a good description of the D meson suppression measured by ALICE experiment, and makes a prediction for B meson suppression and flow.

nucl-th↗

arXiv2012

Heavy quark energy loss and thermalization in hot and dense nuclear matter

We study heavy quark energy loss and thermalization in hot and dense nuclear medium. The diffusion of heavy quarks is calculated via a Langevin equation, both for a static medium as well as for a quark-gluon plasma (QGP) medium generated by a (3+1)-dimensional hydrodynamic model. We investigate how the initial configuration of the QGP and its properties affect the final state spectra and elliptic flow of heavy flavor mesons and non-photonic electrons. It is observed that both the geometric anisotropy of the initial profile and the flow profile of the hydrodynamic medium play important roles in the heavy quark energy loss process and the development of elliptic flow. Within our definition of the thermalization criterion and for reasonable values of the diffusion constant, we observe thermalization times that are longer than the lifetime of the QGP phase.

nucl-th↗

arXiv2012

Probing the QCD Critical Point with Relativistic Heavy-Ion Collisions

We utilize an event-by-event relativistic hydrodynamic calculation performed at a number of different incident beam energies to investigate the creation of hot and dense QCD matter near the critical point. Using state-of-the-art analysis and visualization tools we demonstrate that each collision event probes QCD matter characterized by a wide range of temperatures and baryo-chemical potentials, making a dynamical response of the system to the vicinity of the critical point very difficult to isolate above the background.

nucl-th↗

arXiv2012

A systematic study of the sensitivity of triangular flow to the initial state fluctuations in relativistic heavy-ion collisions

Experimental data from the Relativistic Heavy Ion Collider (RHIC) suggests that the quark gluon plasma behaves almost like an ideal fluid. Due to its short lifetime, many QGP properties can only be inferred indirectly through a comparison of the final state measurements with transport model calculations. Among the current phenomena of interest are the interdependencies between two collective flow phenomena, elliptic and triangular flow. The former is mostly related to the initial geometry and collective expansion of the system whereas the latter is sensitive to the fluctuations of the initial state. For our investigation we use a hybrid transport model based on the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) transport approach using an ideal hydrodynamic expansion for the hot and dense stage. Using UrQMD initial conditions for an Au-Au collision, particles resulting from a collision are mapped into an energy density distribution that is evolved event-by-event with a hydrodynamic calculation. By averaging these distributions over different numbers of events, we have studied how the granularity/smoothness of the distribution affects the initial eccentricity, the initial triangularity, and the resulting flow components. The average elliptic flow in non central collisions is not sensitive to the granularity, while triangular flow is. The triangularity might thus provide a good measure of the amount of initial state fluctuations that is necessary to reproduce the experimental data.

nucl-th↗

arXiv2011

Thermalization of charm quarks in infinite and finite QGP matter

We study the thermalization process of charm quarks in hot and dense matter. The diffusion of heavy quarks is calculated via a Langevin equation, both for a static medium as well as a QGP medium generated by a (3+1)D hydrodynamic model. We define two criteria for the thermalization of the heavy quarks, and observe thermalization times that are longer than the lifetime of the QGP phase for reasonable values of the diffusion constant.

nucl-th↗

arXiv2011

Longitudinal correlation of the triangular flow event plane in a hybrid approach with hadron and parton cascade initial conditions

The longitudinal long-range correlations of the triangular flow event plane angles are calculated in a Boltzmann + hydrodynamics hybrid approach. The potential to disentangle different energy deposition scenarios is explored by utilizing two different transport approaches for the early non-equilibrium evolution. In the hadronic transport approach the particle production in high energy heavy ion reactions is mainly governed by string excitation and fragmentation processes which are absent in the parton cascade approach. We find that in both approaches the initial state shows a strong longitudinal correlation of the event plane angles which is diluted but still persists in the final state momentum space distributions of the produced particles. A ridge-like structure can also be caused by near-collinear gluon radiation in a parton cascade approach and does not necessarily prove longitudinal flux tubes in the initial state.

nucl-th↗

arXiv2011

Elliptic flow in 200 A GeV Au+Au collisions and 2.76 A TeV Pb+Pb collisions: insights from viscous hydrodynamics + hadron cascade hybrid model

Using the newly developed hybrid model VISHNU which connects viscous hydrodynamics with a hadron cascade model, we study the differential and integrated elliptic flow v_2 at different centrality bins for 200 A GeV Au+Au collisions and 2.76 A TeV Pb+Pb collisions. We find that the average Quark Gluon Plasma (QGP) specific shear viscosity eta/s slightly increases from Relativistic Heavy Ion Collider (RHIC) to Large Hadron Collider (LHC) energies. However, a further study assuming different temperature dependencies for (eta/s)_QGP shows that one cannot uniquely constrain the form of (eta/s)_QGP(T) by fitting the spectra and v_2 alone. Based on our current understanding, the question on whether the QGP fluid is more viscous or more perfect in the temperature regime reached by LHC energies is still open.

nucl-th↗

arXiv2011

200 A GeV Au+Au collisions serve a nearly perfect quark-gluon liquid

The specific shear viscosity (eta/s)_QGP of a Quark-Gluon-Plasma (QGP) at temperatures T_c < T < 2T_c is extracted from the centrality dependence of the eccentricity-scaled elliptic flow measured in ultra-relativistic heavy-ion collisions. Coupling viscous fluid dynamics for the QGP with a microscopic transport model for hadronic freeze-out we find that the eccentricity-scaled elliptic flow is a universal function of charged multiplicity per unit overlap area, (1/S)(dN_ch/dy), that depends only on the viscosity but not on the model used for computing the initial fireball eccentricity. Comparing with measurements we find 1 < (4pi)(eta/s)_QGP < 2.5 where the uncertainty range is dominated by model uncertainties for the eccentricity values used to normalize the measured elliptic flow.

nucl-th↗

arXiv2011

Constraining the initial state granularity with bulk observables in Au+Au collisions at $\sqrt{s_{\rm NN}}=200$ GeV

In this paper we conduct a systematic study of the granularity of the initial state of hot and dense QCD matter produced in ultra-relativistic heavy-ion collisions and its influence on bulk observables like particle yields, $m_T$ spectra and elliptic flow. For our investigation we use a hybrid transport model, based on (3+1)d hydrodynamics and a microscopic Boltzmann transport approach. The initial conditions are generated by a non-equilibrium hadronic transport approach and the size of their fluctuations can be adjusted by defining a Gaussian smoothing parameter $σ$. The dependence of the hydrodynamic evolution on the choices of $σ$ and $t_{start}$ is explored by means of a Gaussian emulator. To generate particle yields and elliptic flow that are compatible with experimental data the initial state parameters are constrained to be $σ=1$ fm and $t_{\rm start}=0.5$ fm. In addition, the influence of changes in the equation of state is studied and the results of our event-by-event calculations are compared to a calculation with averaged initial conditions. We conclude that even though the initial state parameters can be constrained by yields and elliptic flow, the granularity needs to be constrained by other correlation and fluctuation observables.

nucl-th↗

arXiv2011

Medium-modified Jets and Initial State Fluctuations as Sources of Charge Correlations Measured at RHIC

We investigate the contribution of medium-modified jets and initial state fluctuations to the asymmetry in charged particle production with respect to the reaction plane. This asymmetry has been suggested as a compelling signature of the Chiral Magnetic Effect in QCD and make a study of "conventional" scenarios for the creation of such charged particle multiplicity fluctuations a timely endeavor. The different pathlength combinations of jets through the medium in non-central heavy ion collisions result in finite correlations of like and different charged particles emitted in the different hemispheres. Our calculation is based on combining jet events from YaJEM (Yet another Jet Energy Loss Model) and a bulk medium evolution. It is found that the jet production probabilities are too small to observe this effect. The influence of initial state fluctuations on this observable is explored using an event-by-event (3+1)d hybrid approach that is based on UrQMD (Ultra-relativistic Quantum Molecular Dynamics) with an ideal hydrodynamic evolution. In this calculation momentum conservation and elliptic flow are explicitly taken into account. The asymmetries in the initial state are translated to a final state momentum asymmetry by the hydrodynamic flow profile. Dependent on the size of the initial state fluctuations the resulting charged particle asymmetries are in qualitative agreement with the preliminary STAR results. The multi-particle correlation as proposed by the PHENIX collaboration can in principle be used to disentangle the different contributions, however is in practice substantially affected by the procedure to subtract trivial resonance decay contributions.

nucl-th↗

arXiv2011

Viscous QCD matter in a hybrid hydrodynamic+Boltzmann approach

A hybrid transport approach for the bulk evolution of viscous QCD matter produced in ultra-relativistic heavy-ion collisions is presented. The expansion of the dense deconfined phase of the reaction is modeled with viscous hydrodynamics while the dilute late hadron gas stage is described microscopically by the Boltzmann equation. The advantages of such a hybrid approach lie in the improved capability of handling large dissipative corrections in the late dilute phase of the reaction, including a realistic treatment of the non-equilibrium hadronic chemistry and kinetic freeze-out. By varying the switching temperature at which the hydrodynamic output is converted to particles for further propagation with the Boltzmann cascade we test the ability of the macroscopic hydrodynamic approach to emulate the microscopic evolution during the hadronic stage and extract the temperature dependence of the effective shear viscosity of the hadron resonance gas produced in the collision. We find that the extracted values depend on the prior hydrodynamic history and hence do not represent fundamental transport properties of the hadron resonance gas. We conclude that viscous fluid dynamics does not provide a faithful description of hadron resonance gas dynamics with predictive power, and that both components of the hybrid approach are needed for a quantitative description of the fireball expansion and its freeze-out.

nucl-th↗

arXiv2011

Implementing the LPM effect in a parton cascade model

Parton Cascade Models (PCM), which describe the full time-evolution of a system of quarks and gluons using pQCD interactions are ideally suited for the description of jet production, including the emission, evolution and energy-loss of the full parton shower in a hot and dense QCD medium. The Landau-Pomeranchuk-Migdal (LPM) effect, the quantum interference of parton wave functions due to repeated scatterings against the background medium, is likely the dominant in-medium effect affecting jet suppression. We have implemented a probabilistic implementation of the LPM effect within the PCM which can be validated against previously derived analytical calculations by Baier et al (BDMPS-Z). Presented at the 6th International Conference on Physics and Astrophysics of Quark Gluon Plasma (ICPAQGP 2010).

hep-ph↗

arXiv2010

Triangular flow in event-by-event ideal hydrodynamics in Au+Au collisions at $\sqrt{s_{\rm NN}}=200A$ GeV

The first calculation of triangular flow $v_3$ in Au+Au collisions at $\sqrt{s_{\rm NN}}=200A$ GeV from an event-by-event (3+1)-d transport+hydrodynamics hybrid approach is presented. As a response to the initial triangularity $ε_3$ of the collision zone, $v_3$ is computed in a similar way to the standard event-plane analysis for elliptic flow $v_2$. It is found that the triangular flow exhibits weak centrality dependence and is roughly equal to elliptic flow in most central collisions. We also explore the transverse momentum and rapidity dependence of $v_2$ and $v_3$ for charged particles as well as identified particles. We conclude that an event-by-event treatment of the ideal hydrodynamic evolution starting with realistic initial conditions generates the main features expected for triangular flow.

nucl-th↗

arXiv2010

Translation of collision geometry fluctuations into momentum anisotropies in relativistic heavy-ion collisions

We develop a systematic framework for the study of the initial collision geometry fluctuations in relativistic heavy-ion collisions and investigate how they evolve through different stages of the fireball history and translate into final particle momentum anisotropies. We find in our event-by-event analysis that only the few lowest momentum anisotropy parameters survive after the hydrodynamical evolution of the system. The geometry of the produced medium is found to be affected by the pre-equilibrium evolution of the medium and the thermal smearing of the discretized event-by-event initial conditions, both of which tend to smear out the spatial anisotropies. We find such effects to be more prominent for higher moments than for lower moments. The correlations between odd and even spatial anisotropy parameters during the pre-equilibrium expansion are quantitatively studied and found to be small. Our study provides a theoretical foundation for the understanding of initial state fluctuations and the collective expansion dynamics in relativistic heavy-ion collisions.

nucl-th↗