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

TL;DR

This study demonstrates that anisotropic escape probability, rather than collective flow, is the primary source of azimuthal anisotropy in transport models for high-energy nuclear collisions.

Contribution

It reveals that parton escape dominates $v_2$ generation, challenging the hydrodynamic paradigm in interpreting anisotropy data.

Findings

Parton $v_2$ mainly arises from anisotropic escape probability.

Hydrodynamic flow contribution to $v_2$ is minimal under realistic conditions.

Unrealistically large cross-sections are needed for flow to dominate.

Abstract

We trace the development of elliptic anisotropy ($v_2$) via parton-parton collision history in two transport models. The parton $v_2$ is studied as a function of the number of collisions of each parton in Au+Au and $d$+Au collisions at $\sqrt{s_{_{\rm NN}}}=200$ GeV. It is found that the majority of $v_2$ comes from the anisotropic escape probability of partons, with no fundamental difference at low and high transverse momenta. The contribution to $v_2$ 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.