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

TL;DR

This study shows that in transport models, the dominant source of azimuthal anisotropy (v2) in heavy-ion collisions is the anisotropic escape of partons, not hydrodynamic flow, challenging existing paradigms.

Contribution

It demonstrates that anisotropic escape probability, rather than collective flow, primarily generates v2 in transport models, questioning hydrodynamic interpretations.

Findings

Majority of v2 arises from anisotropic escape of partons.

Hydrodynamic flow contribution to v2 is small unless cross-section is unrealistically large.

Challenges the hydrodynamic paradigm for interpreting azimuthal anisotropy data.

Abstract

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.