Nuclear Modification of Electron Spectra and Implications for Heavy Quark Energy Loss in $\mathrm{Au}+\mathrm{Au}$ Collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$

The PHENIX experiment has measured midrapidity ($|\eta |<0.35$) transverse momentum spectra ($0.4<p_T<5.0\mathrm{GeV}/c$) of electrons as a function of centrality in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$. Contributions from photon conversions and from light hadron decays, mainly Dalitz decays of ${\pi }^0$ and $\eta$ mesons, were removed. The resulting nonphotonic electron spectra are primarily due to the semileptonic decays of hadrons carrying heavy quarks. Nuclear modification factors were determined by comparison to nonphotonic electrons in $p+p$ collisions. A significant suppression of electrons at high $p_T$ is observed in central $\mathrm{Au}+\mathrm{Au}$ collisions, indicating substantial energy loss of heavy quarks.

Figure 1

(a) Inclusive and nonphotonic electron invariant yields in minimum bias $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$, compared with contributions from all background electron sources included in the cocktail. (b) Invariant yields of electrons from heavy flavor decays for different $\mathrm{Au}+\mathrm{Au}$ centrality classes, scaled by powers of ten for clarity, together with the best fit to the $p+p$ reference scaled with the appropriate nuclear overlap integrals $⟨T_{AA}⟩$. The error bars (brackets) correspond to statistical (systematic) uncertainties.

Figure 2

Nuclear modification factor $R_{AA}$ for electrons from heavy flavor decays as function of $p_T$ in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$ for the different centrality classes. The error bars are statistical only. Error brackets (boxes) indicate the systematic errors related to the uncertainties in the $\mathrm{Au}+\mathrm{Au}$ ($p+p$) measurements. The bands around one show the relative systematic uncertainties in $T_{AA}$. For the most central collisions the ${\pi }^0$ $R_{AA}$ is shown for comparison [2]. For these data, a 13% $p_T$ independent systematic uncertainty (not plotted) represents the uncertainty in $⟨T_{AA}⟩$ and in the ${\pi }^0$ yield normalization.

Figure 3

Nuclear modification factor $R_{AA}$ for electrons from heavy quark decays as function of $p_T$ for the 10% most central $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$ in comparison with predictions from models incorporating charm quark energy loss. Curves (1a)–(1c) and (2a)–(2b) are taken from [24, 25], respectively, where contributions from $B$ meson decays are included in (2a) and (2b) only. Experimental uncertainties are shown as described in Fig. 2.