Tomography of the quark-gluon plasma by charm quarks

Abstract

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 sections 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 fragmentation. The hadronized D mesons then interact with the various hadrons in the hadronic phase with cross sections calculated in an effective lagrangian approach with heavyquark 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.

Single electrons from heavy-flavor mesons in relativistic heavy-ion collisions

“…The hadronized D−mesons then interact with light hadrons in the hadronic phase until freeze out and subsequently undergoes semileptonic decay. We have found that the PHSD approach, which has been applied for charm production in Au+Au collisions at √ s NN =200 GeV [29] and in Pb+Pb collisions at √ s NN =2.76 TeV [30], describes the R AA as well as the v 2 of D−mesons in reasonable agreement with the experimental data from the STAR collaboration [33,34] and from the ALICE collaboration [35,36] when including the initial shadowing effect in the latter case.…”

“…We employ the PYTHIA event generator to produce the heavy-quark pairs and modify their transverse momentum and rapidity such that they are similar to those from the FONLL calculations [32]. In case of p+p collisions at the top RHIC energy of √ s = 200 GeV, the transverse momentum and the rapidity of the charm quark from the PYTHIA event generator are reduced by 10 % and 16 % [29], respectively, and those of the bottom quark are unmodified. The transverse momentum of charm quarks at the invariant energy of √ s = 62.4 GeV is modified to p * T = p T − 0.6 tanh(p T /1.2 GeV), where p T and p * T , respectively, are the original and modified transverse momenta, while the rapidity is reduced by 15 % [32].…”

“…where z is the momentum fraction of the hadron H fragmented from the heavy quark Q while ǫ Q is a fitting parameter which is taken to be ǫ Q = 0.01 for charm [29] and 0.004 for bottom [42]. Figure 3 shows the fragmentation functions of charm and bottom quarks as a function of the hadron momentum fraction z.…”

“…butions of charm and bottom quarks in p+p collisions at √ s = 200 GeV and 62.4 GeV, where the dashed and solid lines are from the tuned PYTHIA and FONLL calculations, respectively. The FONLL calculations are rescaled such that the total cross sections for charm production are 0.8 and 0.12 mb at √ s = 200 GeV and 62.4 GeV, respectively, from the measurement of the STAR collaboration [40] and the interpolation of the PHSD [29]. The ratios of the bottom cross section to the charm cross section are taken to be 0.75 % and 0.145 % at the same energies as in the FONLL calculations [32].…”

“…More recently, the charm production and propagation has been explicitly implemented in the PHSD and detailed studies on the charm dynamics and hadronization/fragmention have been performed at top RHIC and LHC energies in comparison to the available data [29,30]. In the PHSD approach the initial charm and anticharm quarks are produced by using the PYTHIA event generator [31] which is tuned to the transverse momentum and rapidity distributions of charm and anticharm quarks from the Fixed-Order Next-to-Leading Logarithm (FONLL) calculations [32].…”