Towards the dynamical study of heavy-flavor quarks in the Quark-Gluon-Plasma (original) (raw)
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Collisional processes of on-shell and off-shell heavy quarks in vacuum and in the quark-gluon plasma
Physical Review C, 2014
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 quasiparticles (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 → q(g)Q) cross sections which depend on temperature T , energy density ε, the invariant energy √ 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 qQ → qQ as well as gQ → gQ 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.
Physical Review C, 2014
In this study we evaluate the dynamical collisional energy loss of heavy quarks, their interaction rate as well as the different transport coefficients (drag and diffusion coefficients,q, etc). We calculate these different quantities for i) perturbative partons (on-shell particles in the vacuum with fixed and running coupling) and ii) for dynamical quasi-particles (off-shell particles in the QGP medium at finite temperature T with a running coupling in temperature as described by the dynamical quasi-particles model). We use the perturbative elastic (q(g)Q → q(g)Q) cross section for the first case, and the Infrared Enhanced Hard Thermal Loop cross sections for the second. The results obtained in this work demonstrate the effects of a finite parton mass and width on the heavy quark transport properties and provide the basic ingredients for an explicit study of the microscopic dynamics of heavy flavors in the QGP -as formed in relativistic heavy-ion collisions -within transport approaches developed previously by the authors.
Parton propagation in an increasingly strongly interacting quark gluon plasma
The discovery of the jet quenching in central Au + Au collisions at the Relativistic Heavy-ion Collider (RHIC) at Brookhaven National Laboratory has provided clear evidence for the formation of strongly interacting dense matter. It has been predicted to occur due to the energy loss of high energy partons that propagate through the quark gluon plasma. In this paper we investigate the dependence of the parton energy loss due to elastic scatterings in a parton plasma on the value of the strong coupling and its running with the evolution of the system. We analyze different prescriptions for the QCD coupling and calculate the energy and length dependence of the fractional energy loss. Moreover, the quenching factor for light and heavy quarks is estimated. We found that the predicted enhancement of charmed mesons production when compared with π mesons is strongly dependent on the running of the QCD coupling constant.
Heavy quark scattering and quenching in a QCD medium at finite temperature and chemical potential
Physical Review C, 2015
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 quasiparticles (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.
Dilepton production by dynamical quasi-particles in the strongly interacting quark–gluon plasma
Journal of Physics G: Nuclear and Particle Physics, 2011
We address the dilepton production by the constituents of the strongly interacting quark-gluon plasma (sQGP). In order to make quantitative predictions for dilepton rates at experimentally relevant low dilepton mass (O(1 GeV)) and strong coupling (α S ∼ 0.5-1), we take into account nonperturbative spectral functions and self-energies of the quarks, antiquarks and gluons. For this purpose, we use parametrizations of the quark and gluon propagators provided by the dynamical quasi-particle model (DQPM) matched to reproduce lattice quantum chromodynamics (QCD) data. The DQPM describes QCD properties in terms of the single-particle Green's functions and leads to the notion of the constituents of the sQGP being effective quasiparticles, which are massive and have broad spectral functions (due to large interaction rates). By 'dressing' the quark and gluon lines with the effective propagators, we derive the off-shell cross sections for dilepton production in the reactions q +q → l + l − (Born mechanism), q +q → g + l + l − (quark annihilation with the gluon bremsstrahlung in the final state), q(q)+g → q(q)+ l + l − (gluon-Compton scattering), g → q +q +l + l − and q(q) → q(q)+g +l + l − (virtual gluon decay, virtual quark decay). In contrast to previous calculations of these cross sections, we account for virtualities of all the quarks and gluons. We find that finite masses of the effective quasi-particles not only screen the singularities typical of the perturbative cross sections with massless quarks, but also modify the shape of the dilepton production cross sections, especially at low dilepton mass Q and at the edges of the phase space. Finally, we use the calculated mass-dependent cross sections to identify the dependence of the dilepton rates on the spectral function widths of the initial and final quarks and gluons, which has not been estimated so far. The results demonstrate that the multiple partonic scatterings encoded in the broad spectral functions of the dynamical quasi-particles have considerable effect on the dilepton rates.
Nonperturbative Heavy-Quark Diffusion in the Quark-Gluon Plasma
Physical Review Letters, 2008
Heavy quarks (charm and bottom) are valuable probes of the hot and dense matter produced in ultrarelativistic heavy-ion collisions: they are produced in initial hard nucleon-nucleon collisions and subsequently interact with the medium consisting of light quarks and gluons. Data on light hadron spectra in 200 AGeV Au-Au collisions at the Relativistic Heavy-Ion Collider (RHIC) have shown that the produced partonic medium can be described by ideal hydrodynamics, suggestive for a strongly interacting quark-gluon plasma (sQGP) : after the collision the medium appears to equilibrate rapidly building up pressure which is associated with the observed collective flow of hadrons. Heavy quarks, due to their large mass, m Q >>T c (T c ≈180 MeV: critical temperature), are particularly sensitive to the microscopic interaction mechanisms underlying the apparent rapid thermalization. At RHIC the measurement of transverse-momentum (p t ) spectra and elliptic flow, v 2 , of non-photonic electrons [3,4] -originating from the decay of open-charm (D) and -bottom mesons (B) -have lead to the conclusion that heavy quarks interact surprisingly strongly with the medium, largely inheriting its collective-flow pattern via the corresponding drag within the medium. These observations indicate large momentum-diffusion coefficients which can not be accounted for in perturbative QCD (pQCD).
Tomography of the quark-gluon plasma by charm quarks
Physical Review C, 2015
We study charm production in ultra-relativistic heavy-ion collisions by using the Parton-Hadron-String Dynamics (PHSD) transport approach. The initial charm quarks are produced by the Pythia event generator tuned to fit the transverse momentum spectrum and rapidity distribution of charm quarks from Fixed-Order Next-to-Leading Logarithm (FONLL) calculations. The produced charm quarks scatter in the quark-gluon plasma (QGP) with the off-shell partons whose masses and widths are given by the Dynamical Quasi-Particle Model (DQPM) which reproduces the lattice QCD equation-of-state in thermal equilibrium. The relevant cross section are calculated in a consistent way by employing the effective propagators and couplings from the DQPM. Close to the critical energy density of the phase transition, the charm quarks are hadronized into D mesons through coalescence and/or fragmentation depending on transverse momentum. The hadronized D mesons then interact with the various hadrons in the hadronic phase with cross sections calculated in an effective lagrangian approach with heavy-quark spin symmetry. Finally, the nuclear modification factor RAA and the elliptic flow v2 of D 0 mesons from PHSD are compared with the experimental data from the STAR Collaboration for Au+Au collisions at √ sNN =200 GeV. We find that in the PHSD the energy loss of D mesons at high pT can be dominantly attributed to partonic scattering while the actual shape of RAA versus pT reflects the heavy quark hadronization scenario, i.e. coalescence versus fragmentation. Also the hadronic rescattering is important for the RAA at low pT and enhances the D-meson elliptic flow v2.
Nuclear Physics A, 2009
Dilepton production in In + In collisions at 158 A·GeV is studied within the microscopic Parton-Hadron-Strings Dynamics (PHSD) transport approach, which is based on a dynamical quasiparticle model (DQPM) matched to reproduce lattice QCD results in thermodynamic equilibrium. A comparison to the data of the NA60 Collaboration shows that the low mass dilepton spectra are well described by including a collisional broadening of vector mesons, while the spectra in the intermediate mass range are dominated by off-shell quark-antiquark annihilation in the nonperturbative QGP. In particular, the observed softening of the m T spectra at intermediate masses is reproduced.
Evolution of Hot, Dissipative Quark Matter in Relativistic Nuclear Collisions
Non-ideal fluid dynamics with cylindrical symmetry in transverse direction and longitudinal scaling flow is employed to simulate the space-time evolution of the quark-gluon plasma produced in heavy-ion collisions at RHIC energies. The dynamical expansion is studied as a function of initial energy density and initial time. A causal theory of dissipative fluid dynamics is used instead of the standard theories which are acausal. We compute the parton momentum spectra and HBT radii from two-particle correlation functions. We find that, in non-ideal fluid dynamics, the reduction of the longitudinal pressure due to viscous effects leads to an increase of transverse flow and a decrease of the ratio Rout/RsideR_{out}/R_{side}Rout/Rside as compared to the ideal fluid approximation.
Heavy quark dynamics in the QGP: and from RHIC to LHC
Nuclear Physics A, 2011
We study the stochastic dynamics of c and b quarks in the hot plasma produced in nucleus-nucleus collisions at RHIC and LHC, providing results for the nuclear modification factor R AA and the elliptic flow coefficient v 2 of the singleelectron spectra arising from their semi-leptonic decays. The initial QQ pairs are generated using the POWHEG code, implementing pQCD at NLO. For the propagation in the plasma we develop a relativistic Langevin equation (solved in a medium described by hydrodynamics) whose transport coefficients are evaluated through a first-principle calculation. Finally, at T c , the heavy quarks are made hadronize and decay into electrons: the resulting spectra are then compared with RHIC results. Predictions for LHC are also attempted.