Testing the collectivity in large and small colliding systems with test particles

TL;DR

This paper introduces a test-particle method to investigate how collective flow develops in large and small heavy-ion collision systems, revealing the significant role of parton collisions and differences in collectivity persistence.

Contribution

A novel test-particle approach is proposed to analyze the transport dynamics and origin of collectivity in heavy-ion collisions across different system sizes.

Findings

Parton collisions are crucial for developing collectivity.

Large systems exhibit stronger final-state collectivity.

Small systems retain initial-state collectivity more easily.

Abstract

We propose a test-particle method to probe the transport dynamics of the establishment and development of collective flow in large and small systems of heavy-ion collisions. We place test particles as passengers into the partonic medium created by Au$+$Au midcentral collisions at $\sqrt{s_{NN}}$ = 200 GeV and $p$ $+$ Pb central collisions at $\sqrt{s_{NN}}$ = 5.02 TeV, using a multiphase transport model. With the help of test particles in two extreme test cases, we demonstrate that parton collisions play an important role in establishing and developing collectivity in large and small colliding systems. The collectivity established by final state parton collisions is much stronger in large colliding systems compared to small colliding systems. The collectivity from the initial state can persist or survive more easily in small colliding systems than in large colliding systems due to fewer parton collisions. Our study provides a new method to understand the origin of collectivity in large and small colliding systems at the BNL Relativistic Heavy Ion Collider and the CERN Large Hadron Collider.