Anisotropic flow at RHIC: how unique is the number-of-constituent-quark scaling?

The transverse momentum dependence of the anisotropic flow v₂ for π, K, nucleon, Λ, Ξ and Ω is studied for Au+Au collisions at \sqrt{s_NN} = 200\ GeV within two independent string-hadron transport approaches (RQMD and UrQMD). Although both models reach only 60% of the absolute magnitude of the measured v₂, they both predict the particle-type dependence of v₂, as observed by the RHIC experiments: v₂ exhibits a hadron-mass hierarchy (HMH) in the low p_T region and a number-of-constituent-quark (NCQ) dependence in the intermediate p_T region. The failure of the hadronic models to reproduce the absolute magnitude of the observed v₂ indicates that transport calculations of heavy-ion collisions at RHIC must incorporate interactions among quarks and gluons in the early, hot and dense phase. The presence of an NCQ scaling in the string-hadron model results suggests that the particle-type dependences observed in heavy-ion collisions at intermediate p_T might be related to the hadronic cross sections in vacuum rather than to the hadronization process itself.