arXiv subjects

J. Aichelin

Publications and source records attributed to J. Aichelin.

At least 37 records · Page 2Linked to original sources

arXiv2015

Heavy quark scattering and quenching in a QCD medium at finite temperature and chemical potential

The heavy quark collisional scattering on partons of the quark gluon plasma (QGP) is studied in a QCD medium at finite temperature and chemical potential. We evaluate the effects of finite parton masses and widths, finite temperature $T$ and quark chemical potential $μ_q$ on the different elastic cross sections for dynamical quasi-particles (on- and off-shell particles in the QGP medium as described by the dynamical quasi-particles model "DQPM") using the leading order Born diagrams. Our results show clearly the decrease of the $qQ$ and $gQ$ total elastic cross sections when the temperature and the quark chemical potential increase. These effects are amplified for finite $μ_q$ at temperatures lower than the corresponding critical temperature $T_c (μ_q)$. Using these cross sections we, furthermore, estimate the energy loss and longitudinal and transverse momentum transfers of a heavy quark propagating in a finite temperature and chemical potential medium. Accordingly, we have shown that the transport properties of heavy quarks are sensitive to the temperature and chemical potential variations. Our results provide some basic ingredients for the study of charm physics in heavy-ion collisions at Beam Energy Scan (BES) at RHIC and CBM experiment at FAIR.

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arXiv2015

Constraining the nuclear matter equation of state around twice saturation density

Using FOPI data on elliptic flow in Au+Au collisions between 0.4 and 1.5A GeV we extract constraints for the equation of state (EOS) of compressed symmetric nuclear matter using the transport code IQMD by introducing an observable describing the evolution of the size of the elliptic flow as a function of rapidity. This observable is sensitive to the nuclear EOS and a robust tool to constrain the compressibility of nuclear matter up to 2 $ρ_0$.

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arXiv2014

Towards the dynamical study of heavy-flavor quarks in the Quark-Gluon-Plasma

Within the aim of a dynamical study of on- and off-shell heavy quarks Q in the quark gluon plasma (QGP) - as produced in relativistic nucleus-nucleus collisions - we study the heavy quark collisional scattering on partons of the QGP. The elastic cross sections $σ_{q,g-Q}$ are evaluated for perturbative partons (massless on-shell particles) and for dynamical quasi-particles (massive off-shell particles as described by the dynamical quasi-particles model "DQPM") using the leading order Born diagrams. We demonstrate that the finite width of the quasi-particles in the DQPM has little influence on the cross sections $σ_{q,g-Q}$ except close to thresholds. We, furthermore, calculate the heavy quark relaxation time as a function of temperature T within the different approaches using these cross sections.

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arXiv2014

Transport coefficients of heavy quarks around $T_c$ at finite quark chemical potential

The interactions of heavy quarks with the partonic environment at finite temperature $T$ and finite quark chemical potential $μ_q$ are investigated in terms of transport coefficients within the Dynamical Quasi-Particle model (DQPM) designed to reproduce the lattice-QCD results (including the partonic equation of state) in thermodynamic equilibrium. These results are confronted with those of nuclear many-body calculations close to the critical temperature $T_c$. The hadronic and partonic spatial diffusion coefficients join smoothly and show a pronounced minimum around $T_c$, at $μ_q=0$ as well as at finite $μ_q$. Close and above $T_c$ its absolute value matches the lQCD calculations for $μ_q=0$. The smooth transition of the heavy quark transport coefficients from the hadronic to the partonic medium corresponds to a cross over in line with lattice calculations, and differs substantially from perturbative QCD (pQCD) calculations which show a large discontinuity at $T_c$. This indicates that in the vicinity of $T_c$ dynamically dressed massive partons and not massless pQCD partons are the effective degrees-of-freedom in the quark-gluon plasma.

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arXiv2014

Collisional processes of on-shell and off-shell heavy quarks in vacuum and in the Quark-Gluon-Plasma

We study the heavy quark scattering on partons of the quark gluon plasma (QGP) being especially interested in the collisional (elastic) scattering processes of heavy quarks on quarks and gluons. We calculate the different cross sections for perturbative partons (massless on-shell particles in the vacuum) and for dynamical quasi-particles (off-shell particles in the QGP medium as described by the dynamical quasi-particles model "DQPM") using the leading order Born diagrams. Our results show clearly the effect of a finite parton mass and width on the perturbative elastic $(q(g) Q \rightarrow q (g) Q)$ cross sections which depend on temperature $T$, energy density $ε$, the invariant energy $\sqrt{s}$ and the scattering angle $θ$. Our detailed comparisons demonstrate that the finite width of the quasi-particles in the DQPM - which encodes the multiple partonic scattering - has little influence on the cross section for $q Q \rightarrow q Q$ as well as $g Q \rightarrow g Q$ scattering except close to thresholds. Thus when studying the dynamics of energetic heavy quarks in a QGP medium the spectral width of the degrees-of-freedom may be discarded. We have, furthermore, compared the cross sections from the DQPM with corresponding results from hard-thermal-loop (HTL) approaches. The HTL inspired models - essentially fixing the regulators by elementary vacuum cross sections and decay amplitudes instead of properties of the QGP at finite temperature - provide quite different results especially w.r.t. the temperature dependence of the $qQ$ and $gQ$ cross sections (in all settings). Accordingly, the transport properties of heavy quarks will be very different as a function of temperature when compared to DQPM results.

hep-ph↗

arXiv2014

Core - Corona Model analysis of the Low Energy Beam Scan at RHIC (Relativistic Heavy Ion Collider) in Brookhaven (USA)

The centrality dependence of spectra of identified particles in collisions between ultrarelativistic heavy ions with a center of mass energy ($\sqrt{s}$) of 39 and 11.5 $AGeV$ is analyzed in the core - corona model. We show that at these energies the spectra can be well understood assuming that they are composed of two components whose relative fraction depends on the centrality of the interaction: The core component which describes an equilibrated quark gluon plasma and the corona component which is caused by nucleons close to the surface of the interaction zone which scatter only once and which is identical to that observed in proton-proton collisions. The success of this approach at 39 and 11.5 $AGeV$ shows that the physics does not change between this energy and $\sqrt{s}=200~ AGeV$ for which this model has been developed (Aichelin 2008). This presents circumstantial evidence that a quark gluon plasma is also created at center of mass energies as low as 11.5 $AGeV$.

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arXiv2013

System Size and Energy Dependence of Dilepton Production in Heavy-Ion Collisions at SIS Energies

We study the dilepton production in heavy-ion collisions at energies of 1-2 AGeV as well as in proton induced pp, pn, pd and p+A reactions from 1 GeV up to 3.5 GeV. For the analysis we employ three different transport models - the microscopic off-shell Hadron-String-Dynamics (HSD) transport approach, the Isospin Quantum Molecular Dynamics (IQMD) approach as well as the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) approach. We confirm the experimentally observed enhancement of the dilepton yield (normalized to the multiplicity of neutral pions $N_{π^0}$) in heavy-ion collisions with respect to that measured in $NN = (pp+pn)/2$ collisions. We identify two contributions to this enhancement: a) the $pN$ bremsstrahlung which scales with the number of collisions and not with the number of participants, i.e. pions; b) the dilepton emission from intermediate $Δ$'s which are part of the reaction cycles $Δ\to πN ; πN \to Δ$ and $NN\to NΔ; NΔ\to NN$. With increasing system size more generations of intermediate $Δ$'s are created. If such $Δ$ decays into a pion, the pion can be reabsorbed, however, if it decays into a dilepton, the dilepton escapes from the system. Thus, experimentally one observes only one pion (from the last produced $Δ$) whereas the dilepton yield accumulates the contributions from all $Δ$'s of the cycle. We show as well that the Fermi motion enhances the production of pions and dileptons in the same way. Furthermore, employing the off-shell HSD approach, we explore the influence of in-medium effects like the modification of self-energies and spectral functions of the vector mesons due to their interactions with the hadronic environment.

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arXiv2013

Impact of gluon damping on heavy-quark quenching

In this conference contribution, we discuss the influence of gluon-bremsstrahlung damping in hot, absorptive QCD matter on the heavy-quark radiation spectra. Within our Monte-Carlo implementation for the description of the heavy-quark in-medium propagation we demonstrate that as a consequence of gluon damping the quenching of heavy quarks becomes significantly affected at higher transverse momenta.

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arXiv2012

Radiative energy loss of relativistic charged particles in absorptive media

We determine the energy loss spectrum per time-interval of a relativistic charge traversing a dispersive medium. Polarization and absorption effects in the medium are modelled via a complex index of refraction. We find that the spectrum amplitude becomes exponentially damped due to absorption mechanisms. Taking explicitly the effect of multiple scatterings on the charge trajectory into account, we confirm results obtained in a previous work.

hep-ph↗

arXiv2012

Dilepton production in proton-proton and Pb+Pb collisions at sqrt(s_NN)=2.76 TeV

We study e^+e^- pair production in proton-proton and central Pb+Pb collisions at sqrt(s_NN)=2.76 TeV within two models: an extended statistical hadronization model (SHM) and the Parton-Hadron-String Dynamics (PHSD) transport approach. We find that the PHSD calculations roughly agree with the dilepton spectrum from hadronic sources with the 'cocktail' estimates from the statistical hadronization model matched to available data at LHC energies. The dynamical simulations within the PHSD show a moderate increase of the low mass dilepton yield essentially due to the in-medium modification of the rho-meson. Furthermore, pronounced traces of the partonic degrees of freedom are found in the PHSD results in the intermediate mass regime. The dilepton production from the strongly interacting quark-gluon plasma (sQGP) exceeds that from the semi-leptonic decays of open charm and bottom mesons. Additionally, we observe that a transverse momentum cut of 1 GeV/c further suppresses the relative contribution of the heavy meson decays to the dilepton yield, such that the sQGP radiation strongly dominates the spectrum for masses from 1 to 3 GeV, allowing a closer look at the electromagnetic emissivity of the partonic plasma in the early phase of Pb+Pb collisions.

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arXiv2012

$K^-$ and $\bar p$ Spectra for Au+Au Collisions at $\sqrt{s}$ = 200 GeV from STAR, PHENIX and BRAHMS in Comparison to Core-Corona Model Predictions

Based on results obtained with event generators we have launched the core-corona model. It describes in a simplified way but quite successfully the centrality dependence of multiplicity and $ $ of identified particles observed in heavy-ion reaction at beam energies between $\sqrt{s}$ = 17 GeV and 200 GeV. Also the centrality dependence of the elliptic flow, $v_2$, for all charged and identified particles could be explained in this model. Here we extend this analysis and study the centrality dependence of single particle spectra of $K^-$ and ${\bar p}$ measured by the PHENIX, STAR and BRAHMS collaborations. We find that also for these particles the analysis of the spectra in the core-corona model suffers from differences in the data published by the different experimental groups, notably for the pp collisions. As for protons and $K^+$ for each experience the data agree well with the prediction of the core-corona model but the value of the two necessary parameters depends on the experiments. We show as well that the average momentum as a function of the centrality depends in a very sensitive way on the particle species and may be quite different for particles which have about the same mass. Therefore the idea to interpret this centrality dependence as a consequence of a collective expansion of the system, as done in blast way fits may be premature.

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arXiv2012

Radiative and Collisional Energy Loss of Heavy Quarks in Deconfined Matter

We extend our recently advanced model on collisional energy loss of heavy quarks in a quark gluon plasma (QGP) by including radiative energy loss. We discuss the approach and present calculations for PbPb collisions at $\sqrt{s}=2.76 TeV$. The transverse momentum spectra, RAA, and the elliptic flow $v_2$ of heavy quarks have been obtained using the model of Kolb and Heinz for the hydrodynamical expansion of the plasma.

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arXiv2011

Density dependence of symmetry energy and collective transverse in-plane flow

We study the sensitivity of the collective transverse in-plane flow to the symmetry energy and its density dependence at Fermi energies and higher incident energies. We find that collective transverse in-plane flow is sensitive to the symmetry energy and its density dependence at Fermi energies whereas it shows insensitivity at higher incident energies.

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arXiv2011

Sensitivity of the transverse flow towards symmetry energy

We study the sensitivity of transverse flow towards symmetry energy in the Fermi energy region as well as at high energies. We find that transverse flow is sensitive to symmetry energy as well as its density dependence in the Fermi energy region. We also show that the transverse flow can address the symmetry energy at densities about twice the saturation density, however it shows the insensitivity towards the symmetry energy at densities $ρ/ρ_{0}$ $>$ 2. The mechanism for the sensitivity of transverse flow towards symmetry energy as well as its density dependence is also discussed.

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arXiv2011

Isospin effects in the disappearance of flow as a function of colliding geometry

We study the effect of isospin degree of freedom on the balance energy (E$_{bal}$) as well as its mass dependence throughout the mass range 48-270 for two sets of isobaric systems with N/Z = 1 and 1.4 at different colliding geometries ranging from central to peripheral ones. Our findings reveal the dominance of Coulomb repulsion in isospin effects on E$_{bal}$ as well as its mass dependence throughout the range of the colliding geometry. Our results also indicate that the effect of symmetry energy and nucleon-nucleon cross section on E$_{bal}$ is uniform throughout the mass range and throughout the colliding geometry. We also present the counter balancing of nucleon-nucleon collisions and mean field by reducing the Coulomb and the counter balancing of Coulomb and mean filed by removing the nucleon-nucleon collisions.

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arXiv2010

How to determine experimentally the K+ nucleus potential and the K+ N rescattering cross section in a hadronic environment?

Comparing K+ spectra at low transverse momenta for different symmetric collision systems at beam energies around 1 AGeV allows for a direct determination of both the strength of the K+ nucleus potential as well as of the K+N rescattering cross section in a hadronic environment. Other little known or unknown quantities which enter the K+ dynamics, like the production cross sections of K+ mesons or the hadronic equation of state, do not spoil this signal as they cancel by using ratios of spectra. This procedure is based on transport model calculations using the Isospin Quantum Molecular Dynamics (IQMD) model which describes the available data quantitatively.

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arXiv2010

Identified Particle Spectra for Au+Au Collisions at $\sqrt{s}$ = 200 GeV from STAR, PHENIX and BRAHMS in Comparison to Core-Corona Model Predictions

The core-corona model describes quite successfully the centrality dependence of multiplicity and $ $ of identified particles observed in heavy ion reaction at beam energies between $\sqrt{s}$ = 17 GeV and 200 GeV. Also the centrality dependence of the elliptic flow, $v_2$, for all charged and identified particles could be explained in this model. Here we extend this analysis and study the centrality dependence of single particle spectra of identified particles. We concentrate here on protons, antiprotons, kaons and pions which have all been measured by the PHENIX, STAR and BRAHMS collaborations. We find that an analysis of the spectra in the core-corona model suffers from differences in the data published by the different experimental groups, notably for the pp collisions. For each experience the data agree well with the prediction of the core-corona model but the value of the two necessary parameters depends onthe experiments.

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arXiv2010

Dependence of balance energy on isospin degrees of freedom

Using the isospin-dependent quantum molecular dynamics model we study the isospin effects on the disappearance of flow for the reactions of 58Ni+58Ni and 58Fe+58Fe as a function of impact parameter. We found good agreement between our calculations and experimentally measured energy of vanishing flow at all colliding geometries. Our calculations reproduce the experimental data within 5%(10%) at central (peripheral) colliding geometries.

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