Toward a consistent evolution of the quark-gluon plasma and heavy quarks

Heavy-quark observables in ultrarelativistic heavy-ion collisions, like the nuclear modification factor and the elliptic flow, give insight into the mechanisms of high-momentum suppression and low-momentum thermalization of heavy quarks. Here, we present a global study of these two observables within a coupled approach of the heavy-quark propagation in a realistic fluid dynamical medium, $\text{MC@sHQ}+\text{EPOS2}$, and compare with experimental data from the BNL Relativistic Heavy Ion Collider and the CERN Large Hadron Collider experiments. The heavy quarks scatter elastically and inelastically with the quasiparticles of the quark-gluon plasma (QGP), which are represented consistently with the underlying equation of state. We examine two scenarios: first, we interpret the lattice QCD equation of state as a sum of partonic and hadronic contributions and, second, as a gas of massive partonic quasiparticles. It is observed that, independent of their momentum, the energy loss of heavy quarks depends strongly on how the lattice QCD equation of state is translated into degrees of freedom of the QGP.

Figure 1

The fraction of partonic degrees of freedom as given by the EPOS parametrization of the lattice equation of state.

Figure 2

Thermal masses of the quarks and gluons in the QGP within a quasiparticle approach.

Figure 3

The effective reduction ${\lambda }_{\mathrm{m}}$ of the scattering rates for different momenta of the charm quark as a function of temperature for scattering off massive quasi-quarks (upper plot) and quasi-gluons (lower plot).

Figure 4

The effective reduction ${\lambda }_{\mathrm{m}}$ of the scattering rates for different temperatures of the medium as a function of the momentum of the charm quark for scattering off massive quasi-quarks (upper plot) and quasi-gluons (lower plot).

Figure 5

The drag force for the three different representations of the QGP constituents: massless partons (solid), EPOS parametrization of the equation of state (short dashed), and massive quasiparticles (long dashed), as a function of momentum (upper plot) and as a function of medium temperature (lower plot). Results for the purely collisional energy loss (black) and for the $\text{collisional}+\text{radiative}$ (LPM) are shown.

Figure 6

Same as in Fig. 5, but the different scenarios are rescaled with the $K$ factor determined from an optimal description of the central $R_{\mathrm{AA}}$ data at the LHC. See text for details.

Figure 7

Comparison of the $D$ meson $R_{\mathrm{AA}}$ for a QGP consisting of massive quasiparticles (solid lines) and massless partons (dashed lines). The cross sections are not rescaled $\left(K=1\right)$. Purely collisional (orange, light) and $\text{collisional}+\text{radiative}$ (LPM) (black) energy-loss scenarios are shown.

Figure 8

$D$ meson $R_{\mathrm{AA}}$ in central $\sqrt{s_{NN}}=2.76$ TeV $\text{Pb}+\text{Pb}$ collisions. In the upper plot, the EPOS parametrization of the equation of state is used; in the lower plot the scenario with massive quasiparticles is used. Shown are curves for purely collisional and $\text{collisional}+\text{radiative}$ (LPM) energy loss and for the two decoupling-temperatures $T=134$ MeV and $T=168$ MeV. Standard curves refer to the case of massless light partons.

Figure 9

$D$ meson $v_2$ in the 30%–50% most-central $\sqrt{s_{NN}}=2.76$ TeV $\text{Pb}+\text{Pb}$ collisions. In the upper plot the EPOS parametrization of the equation of state is used; in the lower plot the scenario with massive quasiparticles is used. Shown are curves for purely collisional and $\text{collisional}+\text{radiative}$ (LPM) energy loss and for the two decoupling temperatures $T=134$ MeV and $T=168$ MeV. Standard curves refer to the case of massless light partons.

Figure 10

$D$ meson $R_{\mathrm{AA}}$ in central $\sqrt{s_{NN}}=200$ GeV $\text{Au}+\text{Au}$ collisions. In the upper plot the EPOS parametrization of the equation of state is used; in the lower plot the scenario with massive quasiparticles is used. Shown are curves for purely collisional and $\text{collisional}+\text{radiative}$ (LPM) energy loss and for the two decoupling-temperatures $T=134$ MeV and $T=168$ MeV.

Figure 11

$D$ meson $v_2$ in 20%–40% most-central $\sqrt{s_{NN}}=200$ GeV $\text{Au}+\text{Au}$ collisions. In the upper plot, the EPOS parametriz-ation of the equation of state is used; in the lower plot the scenario with massive quasiparticles is used. Shown are curves for purely collisional and $\text{collisional}+\text{radiative}$ (LPM) energy loss and for the two decoupling-temperatures $T=134$ MeV and $T=168$ MeV.

Figure 12

(top) Contribution of quarks to the drag of $c$ quarks as a function of $p$ with parameters corresponding to model C of Ref. [12]. (bottom) Same as top panel but for model E.