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 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(-)(mu(+),mu(-)) angular distribution W(theta)similar to1 + alphacos(2)theta, with alphasimilar or equal to0.35-0.4.