Elliptic flow of identified hadrons in Au+Au collisions at √sNN =7.7–62.4 GeV
Measurements of the elliptic flow, v2, of identified hadrons (π±, K±, K0s, p, p¯, ϕ, Λ, Λ¯¯¯, Ξ−, Ξ¯¯¯+, Ω−, Ω¯¯¯+) in Au+Au collisions at √sNN=7.7, 11.5, 19.6, 27, 39, and 62.4 GeV are presented. The measurements were done at midrapidity using the time-projection chamber and the time-of-flight detectors of the Solenoidal Tracker at RHIC experiment during the beam-energy scan program at Relativistic Heavy Ion Collider. A significant difference in the v2 values for particles and the corresponding antiparticles was observed at all transverse momenta for the first time. The difference increases with decreasing center-of-mass energy, √sNN (or increasing baryon chemical potential, μB), and is larger for the baryons as compared to the mesons. This implies that particles and antiparticles are no longer consistent with the universal number-of-constituent quark (NCQ) scaling of v2 that was observed at √sNN=200 GeV. However, for the selected group of particles (π+, K+, K0s, p, Λ, Ξ−, Ω−) NCQ scaling at (mT−m0)/nq>0.4 GeV/c2 is not violated within ±10%. The v2 values for ϕ 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 pT values.