Elliptic flow of identified hadrons in Au+Au collisions at $\sqrt{s_{NN}}=$ 7.7--62.4 GeV

Abstract

Measurements of the elliptic flow, $v_2$, of identified hadrons ($\ddot{I}{€}^{\pm }$, $K^{\pm }$, $K_s^0$, $p$, $\bar{p}$, $\ddot{I}•$, $\hat{I}›$, $\bar{\hat{I}}›$, $\hat{I}{ž}^{-}$, $\bar{\hat{I}}{ž}^{+}$, $\hat{I}{\text{c◯}}^{-}$, $\bar{\hat{I}}{\text{c◯}}^{+}$) in Au+Au collisions at $\sqrt{s_{NN}}=$ 7.7, 11.5, 19.6, 27, 39 and 62.4 GeV are presented. The measurements were done at mid-rapidity using the Time Projection Chamber and the Time-of-Flight detectors of the STAR experiment during the Beam Energy Scan program at RHIC. A significant difference in the $v_2$ values for particles and the corresponding anti-particles was observed at all transverse momenta for the first time. The difference increases with decreasing center-of-mass energy, $\sqrt{s_{NN}}$ (or increasing baryon chemical potential, $\hat{I}{¼}_B$) and is larger for the baryons as compared to the mesons. This implies that particles and anti-particles are no longer consistent with the universal number-of-constituent quark (NCQ) scaling of $v_2$ that was observed at $\sqrt{s_{NN}}=$ 200 GeV. However, for the group of particles NCQ scaling at $(m_T-m_0)/n_q>$ 0.4 GeV/$c^2$ is not violated within $\pm$10%. The $v_2$ values for $\ddot{I}•$ mesons at 7.7 and 11.5 GeV are approximately two standard deviations from the trend defined by the other hadrons at the highest measured $p_T$ values.