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

The transverse momentum dependence of the anisotropic flow nu(2) for pi, K, nucleon, Lambda, Xi and Omega is studied for Au+Au collisions at root 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 nu(2), they both predict the particle-type dependence of nu(2), as observed by the RHIC experiments: nu(2) 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 nu 2 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.