Dielectron azimuthal anisotropy at mid-rapidity in Au + Au collisions at $\sqrt{s_{NN}}=200$ GeV

We report on the first measurement of the azimuthal anisotropy $\left(v_2\right)$ of dielectrons $(e^{+}{e}^{-}$ pairs) at mid-rapidity from $\sqrt{s_{NN}}=200$ GeV Au + Au collisions with the STAR detector at the Relativistic Heavy Ion Collider (RHIC), presented as a function of transverse momentum $\left(p_T\right)$ for different invariant-mass regions. In the mass region $M_{ee}<1.1$ $\mathrm{GeV}/c^2$ the dielectron $v_2$ measurements are found to be consistent with expectations from ${\pi }^0,\eta ,\omega$, and $\varphi$ decay contributions. In the mass region $1.1<M_{ee}<2.9\mathrm{GeV}/c^2$, the measured dielectron $v_2$ is consistent, within experimental uncertainties, with that from the $c\overline{c}$ contributions.

Figure 1

(a) The normalized $dE/dx$ distribution as a function of momentum from TPC in Au + Au collisions at $\sqrt{s_{NN}}=200$ GeV. (b) $1/\beta$ measurements from TOF versus the momentum from TPC in Au + Au collisions. The $1/\beta$ resolution is 0.011. (c) The normalized $dE/dx$ distribution as a function of momentum with the cut of $|1/\beta -1/{\beta }_{\text{exp}}|<0.025$. An electron band is prominent with the requirement of velocity close to the speed of light from the TOF measurement. Electron candidates whose $n{\sigma }_e$ falls between the lines are selected for further dielectron analysis. (d) The $n{\sigma }_e$ distribution for $0.68<p_T<0.73$ $\mathrm{GeV}/c$ after the cut $|1/\beta -1/{\beta }_{\text{exp}}|<0.025$ is applied. The solid curve represents a multiple Gaussian fit to the $n{\sigma }_e$ distribution. Different components from the fit are also shown. The ${\pi }_{\mathrm{merged}}$ represents contribution from two merged $\pi$ tracks.

Figure 2

The purity of electron candidates as a function of momentum in minimum-bias Au + Au collisions at $\sqrt{s_{NN}}=200$ GeV. In the cross regions, the electron candidates overlap with hadron components in the $dE/dx$ distribution, which results in large uncertainties in the multi-component fit, as shown by the shading around the data points.

Figure 3

(a) The mixed-event unlike-sign and mixed-event like-sign electron pair invariant mass distributions in minimum-bias Au + Au collisions at $\sqrt{s_{NN}}=200$ GeV. (b) The ratio of mixed-event like-sign distribution to mixed-event unlike-sign distribution in minimum-bias Au + Au collisions at $\sqrt{s_{NN}}=200$ GeV. (c) A zoom-in version of panel (b).

Figure 4

The ratio of the mixed-event like-sign distribution to the mixed-event unlike-sign distribution in minimum-bias Au + Au collisions at $\sqrt{s_{NN}}=200$ GeV, as well as specific centrality selections of the collisions.

Figure 5

The ratio of the mixed-event like-sign distribution to the mixed-event unlike-sign distribution in different $p_T$ ranges in minimum-bias Au + Au collisions at $\sqrt{s_{NN}}=200$ GeV.

Figure 6

(a) The electron pair invariant mass distributions for same-event unlike-sign pairs, same-event like-sign pairs, and mixed-event unlike-sign pairs in minimum-bias Au + Au collisions at $\sqrt{s_{NN}}=200$ GeV. The electron candidates are required to be in the range $\left|\eta \right|<1$ and have $p_T$ greater than 0.$2\mathrm{GeV}/c$. The $ee$ pairs are required to be in the rapidity range $|y_{ee}|<1$. Variable bin widths are used for the yields and signal-to-background ratios. (b) The ratio of the same-event like-sign distribution (corrected for the acceptance difference) to the normalized mixed-event unlike-sign distribution in minimum-bias Au + Au collisions at $\sqrt{s_{NN}}=200$ GeV. (c) A zoom-in version of panel (b).

Figure 7

The centrality dependence of the ratio of the same-event like-sign distribution (corrected for the acceptance difference) to the normalized mixed-event unlike-sign distribution in minimum-bias Au + Au collisions at $\sqrt{s_{NN}}=200$ GeV.

Figure 8

The $p_T$ dependence of the ratio of the same-event like-sign distribution (corrected for the acceptance difference) to the normalized mixed-event unlike-sign distribution in minimum-bias Au + Au collisions at $\sqrt{s_{NN}}=200$ GeV.

Figure 9

The centrality dependence of the ratio of the same-event like-sign distribution to the normalized mixed-event like-sign distribution in minimum-bias Au + Au collisions at $\sqrt{s_{NN}}=200$ GeV.

Figure 10

The $p_T$ dependence of the ratio of the same-event like-sign distribution to the normalized mixed-event like-sign distribution in minimum-bias Au + Au collisions at $\sqrt{s_{NN}}=200$ GeV.

Figure 11

The $p_T$ as a function of $M_{ee}$ for dielectron signal without efficiency correction in $\sqrt{s_{NN}}=200$ GeV minimum-bias Au + Au collisions.

Figure 12

(a) The dielectron continuum without efficiency correction in $\sqrt{s_{NN}}=200$ GeV minimum-bias Au + Au collisions. The two-peak structure, as shown in the inset, for $M_{ee}<0.12$ $\mathrm{GeV}/c^2$ is due to photon conversions in the beam pipe and supporting structure. Errors are statistical only. (b) The signal-over-background ratio in $\sqrt{s_{NN}}=200$ GeV minimum-bias Au + Au collisions. The first two data points are not shown for clarity. Errors are statistical.

Figure 13

The event-plane resolution from central to peripheral (left to right) collisions in minimum-bias Au + Au collisions at $\sqrt{s_{NN}}=200$ GeV.

Figure 14

The $v_2$ for the same-event unlike-sign electron pairs (circles) and background (squares) as a function of $M_{ee}$ within STAR acceptance in minimum-bias Au + Au collisions at $\sqrt{s_{NN}}=200$ GeV.

Figure 15

(a) The invariant yields of identified mesons, fit with Tsallis functions [41] in Au + Au collisions at $\sqrt{s_{NN}}=200$ GeV. See text for details. (b)–(d) The $v_2$ of identified mesons, fit with a function for $\sqrt{s_{NN}}=200$ GeV minimum-bias Au + Au collisions. See text for details.

Figure 16

The simulated $v_2$ as a function of $M_{ee}$ from ${\pi }^0,\eta ,\omega$, and $\varphi$ decays within the STAR acceptance in minimum-bias Au + Au collisions at $\sqrt{s_{NN}}=200$ GeV, including the contributions from specific decays. The contribution from $\varphi \to \eta e^{+}{e}^{-}$ is smaller than 1% and is not shown for clarity. The bin width is 20 MeV/$c^2$.

Figure 17

The invariant yield and $v_2$ of electrons from heavy flavor decays [50] fitted with functions in minimum-bias Au + Au collisions at $\sqrt{s_{NN}}=200$ GeV. The spectrum is fit with a function $A{[e^{B\sqrt{{p_T}^2+C}+D({p_T}^2+C)}+\sqrt{{p_T}^2+C}/E]}^F$, where $A,B,C,D,E$, and $F$ are fit parameters. The $v_2$ is fit with the same function as used to parametrize the meson $v_2$ shown in Fig. 15.

Figure 18

The dielectron $v_2$ from the $c\overline{c}$ contribution as a function of $M_{ee}$ and $p_T$ with a completely uncorrelated $c$ and $\overline{c}$.

Figure 19

The dielectron $v_2$ in the ${\pi }^0$ Dalitz decay region (star symbol) as a function of $p_T$ in different centralities from Au + Au collisions at $\sqrt{s_{NN}}=200$ GeV. Also shown are the charged (square) [51] and neutral (diamond) [42] pion $v_2$, and the expected dielectron $v_2$ (dashed curve) from ${\pi }^0$ Dalitz decay. The bars and bands represent statistical and systematic uncertainties, respectively.

Figure 20

The ratio of the measured dielectron $v_2$ in the ${\pi }^0$ Dalitz decay region over the expected dielectron $v_2$ from ${\pi }^0$ Dalitz decay as a function of $p_T$ in different centralities from Au + Au collisions at $\sqrt{s_{NN}}=200$ GeV. The dashed line is a constant fit to the ratio. The bars and bands represent statistical and systematic uncertainties, respectively.

Figure 21

(a)–(f) The dielectron $v_2$ as a function of $p_T$ in minimum-bias Au + Au collisions at $\sqrt{s_{NN}}=200$ GeV for six different mass regions: ${\pi }^0,\eta ,\text{charm}+{\rho }^0,\omega ,\varphi$, and $\text{charm}+\mathrm{thermal}$ radiation. Also shown are the neutral pion [42] $v_2$ and the $\varphi$ meson $v_2$ [44] measured through the decay channel $\varphi \to K^{+}{K}^{-}$. The expected dielectron $v_2$ (dashed curves) from ${\pi }^0,\eta ,\omega$, and $\varphi$ decays in the relevant mass regions are shown in panels (a)–(e) while that from $c\overline{c}$ contributions is shown in panel (f). The bars and bands represent statistical and systematic uncertainties, respectively. The full difference between mixed-event unlike-sign and same-event like-sign background subtraction contributes to the lower bound of the systematic uncertainties, which leads to asymmetric systematic uncertainties in panel (f).

Figure 22

The $p_T$-integrated dielectron $v_2$ as a function of $M_{ee}$ in minimum-bias Au + Au collisions at $\sqrt{s_{NN}}=200$ GeV. Also shown are the corresponding dielectron $v_2$ simulated from ${\pi }^0,\eta ,\omega$, and $\varphi$ decays and a $c\overline{c}$ contribution. The theoretical calculations from hadronic matter and QGP thermal radiation and the sum of these two sources [54] are shown for comparisons. The $v_2$ for hadrons $\pi ,K,p,\varphi$, and $\Lambda$ are also shown for comparison. The bars and boxes represent statistical and systematic uncertainties, respectively. The systematic uncertainty for the first data point is smaller than the size of the marker.