Light nuclei elliptic flow at mid-rapidity in $\sqrt{s_{NN}} = 3.0-3.9$ GeV Au+Au collisions using coalescence model

TL;DR

This study uses a microscopic transport model to analyze light nuclei elliptic flow in heavy-ion collisions, revealing how coalescence and nuclear equation of state influence observed flow patterns and sign changes.

Contribution

It introduces a detailed coalescence model simulation that reproduces the sign change in deuteron elliptic flow and highlights the impact of the nuclear equation of state on flow behavior.

Findings

Successfully reproduces deuteron v2 sign change at 3.2 GeV

Shows coalescence probability depends on azimuthal angle

Demonstrates nuclear EoS stiffness affects flow sign change

Abstract

Light nuclei collective flow is an important probe for understanding their production mechanisms in heavy-ion collisions. The STAR collaboration has reported that the atomic mass number ($A$) scaling of light nuclei elliptic flow $v_2$ is broken at $\sqrt{s_{NN}} = 3.0-3.9$ GeV. The observations reveals that, while protons maintain negative $v_2$ values at mid-rapidity at both 3.0 and 3.2 GeV, light nuclei $v_2$ exhibit a sign change from negative at 3.0 GeV to positive at 3.2 GeV. In this study, we investigate $v_2$ of protons and deuterons in mid-central Au+Au Collisions at $\sqrt{s_{NN}} =$ 3.0, 3.2, 3.5 and 3.9 GeV using the JAM2 microscopic transport model. Deuterons are formed via nucleon coalescence, with the spatial distance ${\Delta R}$ and momentum difference ${\Delta P}$ between constituent protons and neutrons serving as the coalescence criteria. Our calculations successfully reproduce the sign change in deuteron $v_2$ at 3.2 GeV. We observe a strong dependence of nucleon coalescence probability on the azimuthal angle relative to the reaction plane. This effect is primarily driven by the transverse momentum dependence of the mean spatial $\langle {\Delta R} \rangle$ and momentum $\langle {\Delta P} \rangle$ separations between nucleon pairs, which vary with the nucleon azimuthal angle. Moreover, our analysis demonstrates that the stiffness of the nuclear equation of state plays a crucial role in determining the energy dependence of this sign change in deuteron $v_2$ at $\sqrt{s_{NN}}=3.2$ GeV.