Origin of the mass splitting of elliptic anisotropy in a multiphase transport model

TL;DR

This study shows that the mass splitting of elliptic flow ($v_2$) in a multiphase transport model arises mainly from hadronic rescatterings rather than hydrodynamic flow, challenging its use as a unique signature of collective behavior.

Contribution

The paper demonstrates that $v_2$ mass splitting can originate from hadronic rescatterings in a multiphase transport model, not solely from hydrodynamic flow, questioning its role as a definitive signature.

Findings

$v_2$ mass splitting is small immediately after hadronization.

Hadronic rescatterings mainly cause the $v_2$ mass splitting.

No qualitative difference between heavy ion and small system collisions.

Abstract

The mass splitting of elliptic anisotropy ($v_2$) at low transverse momentum is considered as a hallmark of hydrodynamic collective flow. We investigate a multiphase transport (AMPT) model where the $v_2$ is mainly generated by an anisotropic escape mechanism, not of the hydrodynamic flow nature, and where mass splitting is also observed. We demonstrate that the $v_2$ mass splitting in AMPT is small right after hadronization (especially when resonance decays are included); the mass splitting mainly comes from hadronic rescatterings, even though their contribution to the overall charged hadron $v_2$ is small. These findings are qualitatively the same as those from hybrid models that combine hydrodynamics with a hadron cascade. We further show that there is no qualitative difference between heavy ion collisions and small system collisions. Our results indicate that the $v_2$ mass splitting is not a unique signature of hydrodynamic collective flow and thus cannot distinguish whether the elliptic flow is generated mainly from hydrodynamics or the anisotropic parton escape.