Quarkonium polarization in heavy ion collisions as a possible signature of the quark-gluon plasma

The polarization of quarkonium states produced in hadron collisions exhibits strong nonperturbative effects---for example, at small transverse momentum pₜ charmonia appear unpolarized, in sharp contradiction to the predictions of perturbation theory. The quark-gluon plasma is expected to screen away the nonperturbative physics; therefore those quarkonia which escape from the plasma should possess polarization as predicted by perturbative QCD. We estimate the expected J∕ψ polarization at small pₜ, and find that it translates into the asymmetry of the e⁺e^-(μ⁺μ^-) angular distribution W(θ)~1+αcos²θ, with α0.35--0.4.