Observation of an Energy-Dependent Difference in Elliptic Flow between Particles and Antiparticles in Relativistic Heavy Ion Collisions

Elliptic flow ($v_2$) values for identified particles at midrapidity in $\mathrm{Au}+\mathrm{Au}$ collisions, measured by the STAR experiment in the beam energy scan at RHIC at $\sqrt{s_{NN}}=7.7–62.4\mathrm{GeV}$, are presented. A beam-energy-dependent difference of the values of $v_2$ between particles and corresponding antiparticles was observed. The difference increases with decreasing beam energy and is larger for baryons compared to mesons. This implies that, at lower energies, particles and antiparticles are not consistent with the universal number-of-constituent-quark scaling of $v_2$ that was observed at $\sqrt{s_{NN}}=200\mathrm{GeV}$.

Figure 1

The elliptic flow $v_2$ of protons and antiprotons as a function of the transverse momentum $p_T$ for 0%–80% central $\mathrm{Au}+\mathrm{Au}$ collisions. The lower panels show the difference in $v_2(p_T)$ between the particles and antiparticles. The solid curves are fits with a horizontal line. The shaded areas depict the magnitude of the systematic errors.

Figure 2

The difference in $v_2$ between particles ($X$) and their corresponding antiparticles ($\overline{X}$) (see legend) as a function of $\sqrt{s_{NN}}$ for 0%–80% central $\mathrm{Au}+\mathrm{Au}$ collisions. The dashed lines in the plot are fits with a power-law function. The error bars depict the combined statistical and systematic errors.

Figure 3

The upper panels depict the elliptic flow $v_2$ as a function of reduced transverse mass ($m_T-m_0$) for particles, (a) and (b), and antiparticles, (c) and (d), in 0%–80% central $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=11.5$ and 62.4 GeV. Simultaneous fits to the mesons except the pions are shown as the dashed lines. The difference of the baryon $v_2$ and the meson fits are shown in the lower panels.

Figure 4

The number-of-constituent-quark scaled elliptic flow $(v_2/n_q)\mathbf{(}(m_T-m_0)/n_q\mathbf{)}$ for 0%–80% central $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=11.5$ and 62.4 GeV for selected particles, (a) and (b), and a direct comparison between selected baryons and antibaryons, (c) and (d). The dashed lines are simultaneous fits [30] to $p$, $\overline{p}$, $\Lambda$, and $\overline{\Lambda }$ at a given energy. The lower panels depict the differences to the fits. Some data points for $\varphi$ are out of the plot range in the lower panels.