Centrality dependence of identified particle elliptic flow in relativistic heavy ion collisions at $\sqrt{s_{NN}}=7.7–62.4$ GeV

Elliptic flow $\left(v_2\right)$ values for identified particles at midrapidity in Au + Au collisions measured by the STAR experiment in the Beam Energy Scan at the Relativistic Heavy Ion Collider at $\sqrt{s_{NN}}=7.7–62.4$ GeV are presented for three centrality classes. The centrality dependence and the data at $\sqrt{s_{NN}}=14.5$ GeV are new. Except at the lowest beam energies, we observe a similar relative $v_2$ baryon-meson splitting for all centrality classes which is in agreement within 15% with the number-of-constituent quark scaling. The larger $v_2$ for most particles relative to antiparticles, already observed for minimum bias collisions, shows a clear centrality dependence, with the largest difference for the most central collisions. Also, the results are compared with a multiphase transport (AMPT) model and fit with a blast wave model.

Figure 1

Elliptic flow $v_2$ as a function of $p_{\text{T}}$ for minimum bias data (0%–80% centrality) at $\sqrt{s_{NN}}=14.5$ GeV for identified particles. (a) Positively charged particles. (b) Negatively charged particles. (c) Neutral particles. The systematic errors are shown by the short error bars with caps. The lines connect the points.

Figure 2

The subevent plane resolution for several beam energies versus centrality with 5% being the most central.

Figure 3

The elliptic flow $v_2$ of identified particles $({\pi }^{+},K^{+},K_s^0,p,\varphi ,\mathrm{\Lambda },{\mathrm{\Xi }}^{-},{\mathrm{\Omega }}^{-})$ as a function of $m_{\text{T}}-m_0$, for 0%–10%, 10%–40%, and 40%–80% central Au + Au collisions at $\sqrt{s_{NN}}=7.7$, 11.5, 14.5, 19.6, 27, 39, and 62.4 GeV. The lines show simultaneous fits to baryons and mesons with Eq. (1). The systematic errors are shown by the hooked error bars.

Figure 4

The elliptic flow $v_2$ of identified antiparticles $({\pi }^{-},K^{-},K_s^0,\overline{p},\varphi ,\overline{\mathrm{\Lambda }},{\overline{\mathrm{\Xi }}}^{+},{\overline{\mathrm{\Omega }}}^{+})$ as a function of $m_{\text{T}}-m_0$, for 0%–10%, 10%–40%, and 40%–80% central Au + Au collisions at $\sqrt{s_{NN}}=7.7$, 11.5, 14.5, 19.6, 27, 39, and 62.4 GeV. The lines show simultaneous fits to baryons and mesons with Eq. (1). The systematic errors are shown by the hooked error bars.

Figure 5

The ratio of $v_2$ between baryons (B) and mesons (M) of particles $\left(X\right)$ and antiparticles $\left(\overline{X}\right)$ as a function of $\sqrt{s_{NN}}$ for 0%–10%, 10%–40% and 40%–80% central Au + Au collisions. The values of baryons and mesons are taken from the fit lines in Figs. 3 and 4 with Eq. (1) at the appropriate values of $m_{\text{T}}-m_0$. See text for details. The open points are for antiparticles and the closed points for particles.

Figure 6

(a) The difference in $v_2$ between particles $\left(X\right)$ and their corresponding antiparticles $\left(\overline{X}\right)$ (see legend) as a function of $\sqrt{s_{NN}}$ for 10%–40% central Au + Au collisions. (b) The difference in $v_2$ between protons and antiprotons as a function of $\sqrt{s_{NN}}$ for 0%–10%, 10%–40% and 40%–80% central Au + Au collisions. (c) The relative difference. The systematic errors are shown by the hooked error bars. The dashed lines in the plot are fits with a power-law function.

Figure 7

The $v_2$ difference between protons and antiprotons (and between ${\pi }^{+}$ and ${\pi }^{-})$ for 10%–40% centrality Au+Au collisions at 7.7, 11.5, 14.5, and 19.6 GeV. The $v_2\left\{\mathrm{BBC}\right\}$ results were slightly shifted horizontally.

Figure 8

Elliptic flow $v_2$ as a function of $p_{\text{T}}$ for $K_s^0$ data at $\sqrt{s_{NN}}=39$ GeV for 10%–40% centrality. The curves are for AMPT default and AMPT string melting with cross sections of 1.5, 3.0, and 6.0 mb.

Figure 9

Elliptic flow $v_2$ as a function of $p_{\text{T}}$ for particles and antiparticles. The symbols show the experimental data. The error bars are mostly smaller than the points. The lines, with the same color code and the same order, show the AMPT string melting calculations with a cross section 1.5 mb. Antiparticles are on the left for three centrality bins. Particles are on the right for three beam energies.

Figure 10

Elliptic flow $v_2$ as a function of $p_{\text{T}}$ for protons minus antiprotons at $\sqrt{s_{NN}}=27$ GeV for three centralities. The curves are for AMPT string melting with cross sections of 1.5 mb. The symbols are data.

Figure 11

Elliptic flow $v_2$ as a function of $p_{\text{T}}$ for particles and antiparticles. The data are shown by symbols. On the left side are blast wave fits (lines) for three centrality bins for $\sqrt{s_{NN}}=27$ GeV. On the right side are blast wave fits for three beam energies for 10%–40% centrality. Antiparticles are on the left, particles on the right. The lines are the same color and in the same order as the points. The dashed lines for $\pi$ and $\varphi$ are not fits, but predictions based on the other fits. The error bars depict the combined statistical and systematic errors.

Figure 12

The transverse radial velocity parameter $\beta$ as a function of $\sqrt{s_{NN}}$ for Au + Au collisions for three centralities and three assumed temperatures. The circles are for 120 MeV, open red circles for antiparticles, and closed black circles for particles. The error bars seen at low beam energies depict the combined statistical and systematic errors. The solid red and black lines show the result when the $s_2$ parameter is fixed at 0.02 and the temperature held at 120 MeV.