Collective flow and the fluid behavior in p/d/$^3$He+Au collisions at $\sqrt{s_{NN}} = 200$ GeV

TL;DR

This study uses hybrid hydrodynamic models to analyze collective flow in small collision systems at RHIC, highlighting the importance of initial geometry and the limits of hydrodynamics validity.

Contribution

It demonstrates the capability of the iEBE-VISHNU model with Trento initial conditions to describe flow data and emphasizes the importance of considering hydrodynamics validity in small systems.

Findings

Hydrodynamic models can fit flow data with fine-tuned parameters.

Hydrodynamics may break down when the Knudsen number exceeds one.

Initial geometry fluctuations significantly influence collective flow.

Abstract

By varying the intrinsic initial geometry, the p/d/$^3$He+Au collisions at the Relativistic Heavy Ion Collider (RHIC) provide a unique opportunity to understand the collective behavior and probe the possible sub-nucleon fluctuations in small systems. In this paper, we employ the hybrid model iEBE-VISHNU with Trento initial conditions to study the collective flow and the fluid behavior in p/d/$^3$He+Au collisions. With fine-tuned parameters, iEBE-VISHNU can describe the $v_2(p_T)$ and $v_3(p_T)$ data from the PHENIX and STAR collaborations. However, for some certain parameter sets with initial sub-nucleon fluctuation, the hydrodynamic simulations have already beyond their limits with the average Knudsen number $\langle K_n \rangle$ obviously larger than one. Our calculations demonstrate that, for a meaningful evaluation of the fluid behavior in the small systems, model simulations should also pay attention to the validity range of hydrodynamics.