The polarization of quarkonium states produced in hadron collisions exhibits strong nonperturbative effects—for example, at small transverse momentum $p_t$ 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 $J∕\psi$ polarization at small $p_t$, and find that it translates into the asymmetry of the $e^{+}{e}^{-}\left({\mu }^{+}{\mu }^{-}\right)$ angular distribution $W\left(\theta \right)\sim 1+\alpha {\cos }^2\theta$, with $\alpha \simeq 0.35–0.4$.

• Received 8 July 2003

DOI:https://doi.org/10.1103/PhysRevC.68.061902