How AMPT generates large elliptic flow with small cross sections

TL;DR

This paper explains how the AMPT model produces high elliptic flow in heavy-ion collisions despite small partonic cross sections, resolving a longstanding opacity puzzle at RHIC.

Contribution

It identifies key features in initial conditions and dynamics within AMPT that enable it to generate large elliptic flow with minimal partonic interactions.

Findings

AMPT achieves high v2 with small cross sections

Comparison with MPC clarifies the role of initial conditions

Features of AMPT's partonic stage circumvent the opacity puzzle

Abstract

We resolve the long-standing open question of how the transport model AMPT manages to generate sufficiently high elliptic flow (v2) in A+A reactions with only few-millibarn 2->2 partonic cross sections - in apparent contradiction with an early study by Molnar and Gyulassy. Through detailed comparisons with the covariant Molnar's Parton Cascade (MPC), we pinpoint which features of initial conditions, interactions, and dynamics encoded in the partonic stage of AMPT allow it to circumvent the "opacity puzzle" at RHIC.