An analysis of the spin density matrix of quarkonium in heavy ion collisions

TL;DR

This paper applies spin density matrix analysis to quarkonium in heavy ion collisions, suggesting the data indicates a maximally impure state and proposing a specific measurement to test this conclusion.

Contribution

It extends previous techniques to analyze quarkonium spin alignment, highlighting the importance of azimuthal angle dependence measurements.

Findings

Data suggests a maximally impure density matrix.

Current models align with statistical freeze-out assumptions.

Proposes a crucial measurement involving azimuthal angle dependence.

Abstract

In this addendum to [1], we apply the techniques developed in that paper to the $J/\Psi$ and $\Upsilon$ spin alignement measurements in [2]. We argue that while the data points to a maximally impure density matrix, consistent with Cooper-Frye/Statistical model freeze-out, a measurement of the dependence of the $\phi$-sensitive coefficients on the azimuthal angle with respect to the reaction plane would be the crucial test of this conclusion.