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It is commonly believed that azimuthal anisotropies in relativistic heavy ion collisions are generated by hydrodynamic evolution of the strongly interacting quark-gluon plasma. Here we use transport models to study how azimuthal anisotropies depend on the number of collisions that each parton suffers. We find that the majority of ν 2 comes from the anisotropic escape of partons, not from the parton collective flow, for semi-central Au+Au collisions at 200A GeV. As expected, the fraction of ν 2 from the anisotropic particle escape is even higher for smaller systems such as d+Au. Our transport model results also confirm that azimuthal anisotropies would be dominated by hydrodynamic flow at unrealistically-high parton cross sections. Our finding thus naturally explains the similarity of azimuthal anisotropies in small and large systems; however, it presents a challenge to the paradigm of anisotropic flow. [ABSTRACT FROM AUTHOR]

We trace the development of azimuthal anisotropy ( v n , n = 2 , 3 ) via parton–parton collision history in two transport models. The parton v n is studied as a function of the number of collisions of each parton in Au + Au and d + Au collisions at s NN = 200 GeV . It is found that the majority of v n comes from the anisotropic escape probability of partons, with no fundamental difference at low and high transverse momenta. The contribution to v n from hydrodynamic-type collective flow is found to be small. Only when the parton–parton cross-section is set unrealistically large does this contribution start to take over. Our findings challenge the current paradigm emerged from hydrodynamic comparisons to anisotropy data. [ABSTRACT FROM AUTHOR]