Anisotropic parton escape is the dominant source of azimuthal anisotropy in transport models

We trace the development of elliptic anisotropy (v2) via parton-parton collision history in two transport models. The parton v2 is studied as a function of the number of collisions of each parton in Au+Au and d+Au collisions at sNN =200 GeV. It is found that the majority of v2 comes from the anisotropic escape probability of partons, with no fundamental difference at low and high transverse momenta. The contribution to v2 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.