Observation of an energy-dependent difference in elliptic flow between particles and anti-particles in relativistic heavy ion collisions

Elliptic flow ($v_2$) values for identified particles at mid-rapidity in Au+Au collisions, measured by the STAR experiment in the Beam Energy Scan at RHIC at $\sqrt{s_{NN}}=$ 7.7–62.4 GeV, are presented. A beam-energy dependent difference of the values of $v_2$ between particles and corresponding anti-particles 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 anti-particles are not consistent with the universal number-of-constituent-quark (NCQ) scaling of $v_2$ that was observed at $\sqrt{s_{NN}}=$ 200 GeV.