APH N.S., Heavy Ion Physics 16/1-4 (2002) 141-152 HEAVY ION PHYSICS @Akademiai Kiad6 A ° Polarization in Ultrarelativistic Nucleus-Nucleus Collisions Alejandro Ayala,' Eleazar Cuautle,' Gerardo Herrera- and Luis Manuel Montafio? 1 Instituto de Ciencias Nucleares, Universidad Nacional Aut6noma de Mexico Apartado Postal 70-543, Mexico Distrito Federal 04510, Mexico 2 Centro de Investigaci6n y de Estudios Avanzados Apartado Postal 14-740, Mexico Distrito Federal 07000, Mexico Received 15 October 2001 Abstract. We study the polarization of A 0 's produced in ultrarelativistic heavy-ion collisions. We argue that when the density of participants in the re- action is below the critical value for the production of a Quark-Gluon Plasma (QGP), recombination processes dominate the A 0 production cross section and hence, Ao's are polarized. However, when the density of participants is above the critical value for QGP formation, coalescence processes dominate A 0 pro- duction, giving rise to a reduction in the A 0 polarization. For densities below the critical density for QGP production, we describe A 0 polarization in terms of the DeGrand-Miettinen model and correct for the effects introduced by mul- tiple scattering of the produced A 0 within the nuclear environment. Keywords: ultrarelativistic heavy-ion collisions, A 0 polarization PAGS: 13.88.+e, 13.75.Ev 1. Introduction High-energy nuclear collisions are expected to produce states of matter subject to extreme conditions, the most exciting of such is the Quark-Giuon Plasma (QGP) where the elementary QCD degrees of freedom are not confined within the volume of a hadron but rather, over a larger volume on the order of the nuclear size. To identify the phase transition from ordinary nuclear matter to the QGP in this kind of collisions, a number of experimental observables have been proposed [1]. At the same time, it has also been recognized that no single signal can, by itself, provide clear cut evidence for the existence of the QGP and that it is only through the combined analysis of all possible signals that the production of such state of matter 1219-7580/02/ $ 5.00 © 2002 Akademiai Kiad6, Budapest