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

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.