Energy Loss and Flow of Heavy Quarks in $\mathrm{Au}+\mathrm{Au}$ Collisions at $\sqrt{s_{\mathrm{NN}}}=200\mathrm{GeV}$

The PHENIX experiment at the BNL Relativistic Heavy Ion Collider (RHIC) has measured electrons with $0.3<p_T<9\mathrm{GeV}/c$ at midrapidity ($|y|<0.35$) from heavy-flavor (charm and bottom) decays in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{\mathrm{NN}}}=200\mathrm{GeV}$. The nuclear modification factor $R_{\mathrm{AA}}$ relative to $p+p$ collisions shows a strong suppression in central $\mathrm{Au}+\mathrm{Au}$ collisions, indicating substantial energy loss of heavy quarks in the medium produced at RHIC energies. A large azimuthal anisotropy $v_2$ with respect to the reaction plane is observed for $0.5<p_T<5\mathrm{GeV}/c$ indicating substantial heavy-flavor elliptic flow. Both $R_{\mathrm{AA}}$ and $v_2$ show a $p_T$ dependence different from those of neutral pions. A comparison to transport models which simultaneously describe $R_{\mathrm{AA}}(p_T)$ and $v_2(p_T)$ suggests that the viscosity to entropy density ratio is close to the conjectured quantum lower bound, i.e., near a perfect fluid.

Figure 1

Invariant yields of electrons from heavy-flavor decays for different $\mathrm{Au}+\mathrm{Au}$ centrality classes and for $p+p$ collisions, scaled by powers of 10 for clarity. The solid lines are the result of a FONLL calculation normalized to the $p+p$ data [18] and scaled with $⟨T_{\mathrm{AA}}⟩$ for each $\mathrm{Au}+\mathrm{Au}$ centrality class. The inset shows the ratio of heavy-flavor to background electrons for minimum bias $\mathrm{Au}+\mathrm{Au}$ collisions. Error bars (boxes) depict statistical (systematic) uncertainties.

Figure 2

$R_{\mathrm{AA}}$ of heavy-flavor electrons with $p_T$ above 0.3 and $3\mathrm{GeV}/c$ and of ${\pi }^0$ with $p_T>4\mathrm{GeV}/c$ as function of centrality given by $N_{\mathrm{part}}$. Error bars (boxes) depict statistical (point-by-point systematic) uncertainties. The right (left) box at $R_{\mathrm{AA}}=1$ shows the relative uncertainty from the $p+p$ reference common to all points for $p_T>0.3(3)\mathrm{GeV}/c$.

Figure 3

(a) $R_{\mathrm{AA}}$ of heavy-flavor electrons in 0%–10% central collisions compared with ${\pi }^0$ data [6] and model calculations (curves I [30], II [31], and III [32]). The box at $R_{\mathrm{AA}}=1$ shows the uncertainty in $T_{\mathrm{AA}}$. (b) $v_2^{\mathrm{HF}}$ of heavy-flavor electrons in minimum bias collisions compared with ${\pi }^0$ data [29] and the same models. Errors are shown as in Fig. 2.