Strangeness enhancement in heavy ion collisions - evidence for quark-gluonmatter?

The centrality dependence of (multi-)strange hadron abundances is studied f or Pb(158 A GeV)Pb reactions and compared to p(158 GeV)Pb collisions. The m icroscopic transport model UrQMD is used for this analysis. The predicted L ambda/pi(-), Xi(-)/pi(-) and Omega(-)/pi(-) ratios are enhanced due to resc attering in central Pb-Pb collisions as compared to peripheral Pb-Pb or p-P b collisions. However, the enhancement is substantially smaller than observ ed experimentally. The enhancement depends strongly on the kinematical cuts . The maximum enhancement is predicted around midrapidity. For Lambda's, st rangeness suppression is predicted at projectile/target rapidity. For Omega 's, the predicted enhancement can be as large as one order of magnitude. Co mparisons of Pb-Pb data to proton induced asymmetric (p-A) collisions are h ampered due to the predicted strong asymmetry in the various rapidity distr ibutions of the different (strange) particle species. In p-Pb collisions, s trangeness is locally (in rapidity) not conserved. The present comparison to the data of the WA97 and NA49 Collaborations clea rly supports the suggestion that conventional (free) hadronic scenarios are unable to describe the observed high (anti-)hyperon yields in central coll isions. A reduction of the constituent quark masses to the current quark ma sses m(s) similar to 230 MeV, m(q) similar to 10 MeV, as motivated by chira l symmetry restoration, yields hyperon production close to the experimental ly observed high values. An ad hoc overall increase of the color electric f ield strength (effective string tension of kappa = 3 GeV/fm) yields similar results. It has been suggested that these findings might be interpreted as a signal of a phase of nearly massless particles. (C) 1999 Published by El sevier Science B.V. All rights reserved.