$J/\psi$ polarization in relativistic heavy ion collisions

TL;DR

This paper investigates the polarization of $J/\psi$ particles in relativistic heavy ion collisions, analyzing how production mechanisms and medium effects influence polarization across different conditions and frames.

Contribution

It provides a comprehensive analysis of $J/\psi$ polarization considering both initial production and regeneration, offering a baseline for studying electromagnetic and vorticity effects.

Findings

High $p_T$ $J/\psi$ polarization similar to $pp$ collisions.

Low $p_T$ $J/\psi$ polarization influenced by charm quark polarization in QGP.

Regeneration dominates at low $p_T$, affecting polarization.

Abstract

Understanding the polarization property of $J/\psi$ is critical to constrain its production mechanism. In addition, the polarization of $J/\psi$ can reveal the impact of strong electromagnetic and vorticity fields in relativistic heavy ion collisions. In this study, we analyzed the yield and polarization of $J/\psi$ in relativistic heavy ion collisions at different centrality and transverse momentum regions, using three different reference frames: the Collins-Soper frame, the helicity frame, and the event plane frame. The polarization of initially produced $J/\psi$ is determined by the NRQCD calculation and is similar to that of $pp$ collisions. However, both unpolarization and transverse polarization are considered for the regenerated $J/\psi$. Our results indicate that the polarization at high $p_T$ is similar to that observed in $pp$ collisions. However, at low $p_T$, where regenerated $J/\psi$ dominates, it is likely that the polarized charm quarks in the rotational QGP medium are responsible for this phenomenon. Our study supplies a baseline for future research on the effects of strong electromagnetic and vorticity fields on $J/\psi$ polarization.