Elliptic flow of electrons from heavy-flavor hadron decays in Au + Au collisions at $\sqrt{s_{NN}}=200$, 62.4, and 39 GeV

We present measurements of elliptic flow ($v_2$) of electrons from the decays of heavy-flavor hadrons ($e_{\mathrm{HF}}$) by the STAR experiment. For Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV we report $v_2$, for transverse momentum ($p_T$) between 0.2 and 7 $\mathrm{GeV}/c$, using three methods: the event plane method ($v_2\left\{\mathrm{EP}\right\}$), two-particle correlations ($v_2\left\{2\right\}$), and four-particle correlations ($v_2\left\{4\right\}$). For Au+Au collisions at $\sqrt{s_{NN}}=62.4$ and 39 GeV we report $v_2\left\{2\right\}$ for $p_T<2\mathrm{GeV}/c$. $v_2\left\{2\right\}$ and $v_2\left\{4\right\}$ are nonzero at low and intermediate $p_T$ at 200 GeV, and $v_2\left\{2\right\}$ is consistent with zero at low $p_T$ at other energies. The $v_2\left\{2\right\}$ at the two lower beam energies is systematically lower than at $\sqrt{s_{NN}}=200$ GeV for $p_T<1\mathrm{GeV}/c$. This difference may suggest that charm quarks interact less strongly with the surrounding nuclear matter at those two lower energies compared to $\sqrt{s_{NN}}=200$ GeV.

Figure 1

Examples of $n{\sigma }_e$ distribution with fits for different hadronic components for minimum-bias Au+Au collisions at $\sqrt{s_{NN}}=62.4$ GeV at (a) low and (b) high momenta.

Figure 2

(a) Electron purity and (b) photonic electron tagging efficiency. The bands show the combined systematic and statistical uncertainties. Centrality classes are indicated in the plot.

Figure 3

Electron pair invariant mass distribution for electrons with $1.2<p_T<2\mathrm{GeV}/c$ for the 0–60 % most central Au+Au collisions at (a) $\sqrt{s_{NN}}=39$ GeV, (b) $\sqrt{s_{NN}}=62.4$ GeV, and (c) $\sqrt{s_{NN}}=200$ GeV.

Figure 4

Signal-to-background ratio for electrons from heavy-flavor hadron decays in Au+Au collisions at $\sqrt{s_{NN}}=200$, 62.4, and 39 GeV in events with minimum-bias (“Min-Bias”) and high tower (“High-Tower”) triggers. The error bars represent the statistical uncertainty, and the brackets represent the systematic uncertainties. See text for details.

Figure 5

Inclusive and photonic electron $v_2\left\{2\right\}$ and $v_2\left\{4\right\}$ at $\sqrt{s_{NN}}=200$, 62.4, and 39 GeV. The error bars on the inclusive electron $v_2$ represent the statistical uncertainty. See text for details.

Figure 6

(a) Elliptic flow $v_2$ of electrons from heavy-flavor hadron decays at $\sqrt{s_{NN}}=200$ GeV compared to PHENIX measurements [20]. (b) $e_{\mathrm{HF}}$ $v_2\left\{2\right\}$ at 200, 62.4, and 39 GeV. The error bars represent the statistical uncertainty, and the brackets represent the systematic uncertainties. Nonflow in (a) was estimated based on $e_{\mathrm{HF}}$-hadron correlations [65] for $p_T>2.5$ $\mathrm{GeV}/c$ and pythia for $p_T<2.5$ $\mathrm{GeV}/c$. The band includes the combined systematic and statistical uncertainties. The curves in (b) show TMatrix model calculations for $\sqrt{s_{NN}}=62.4$ GeV [66] and 200 GeV [67].

Figure 7

The $e_{\mathrm{HF}}$ elliptic flow $v_2\left\{2\right\}$ and $v_2\left\{4\right\}$ at $\sqrt{s_{NN}}=200$ GeV (minimum bias) from Fig. 6 compared to model calculations.