One of the goal of RHIC Beam Energy Scan program is to search for the phase transition boundary between partonic and hadronic matters. Azimuthal anisotropy is expected to be sensitive to the degree of freedom of the produced matter in the early stage of high energy nuclear collisions, which makes it an important probe to QCD phase transition. In this paper, we present the results of mid-rapidity elliptic flow measurements in Au + Au collisions at (SNN)~(1/2) = 7.7, 11.5 and 39 GeV from the STAR experiment at RHIC. The energy evolution of Number-of-Constituent-Quark (NCQ) scaling will be tested with the V_2 of identified hadrons (π~(±), K~(±), K_s~o, p, p, ?, Λ, Λ, Ξ and Ξ~+ ). A significant difference in V_2 is observed between particles and corresponding anti-particles at the lowest energies of the beam energy scan. The implications on partonic-hadronic phase transition are discussed.
展开▼
