Abstract:
The centrality dependence of (multi-)strange hadron abundances is studied for Pb(158 A GeV)Pb reactions and compared to p(158 GeV)Pb collisions. The microscopic transport model UrQMD is used for this analysis. The predicted Λ/π-, Ξ-/π- and Ω-/π- ratios are enhanced due to rescattering in central Pb-Pb collisions as compared to peripheral Pb-Pb or p-Pb collisions. However, the enhancement is substantially smaller than observed experimentally. The enhancement depends strongly on the kinematical cuts. The maximum enhancement is predicted around midrapidity. For A's, strangeness suppression is predicted at projectile/target rapidity. For Λ's, the predicted enhancement can be as large as one order of magnitude. Comparisons of Pb-Pb data to proton induced asymmetric (p-A) collisions are hampered due to the predicted strong asymmetry in the various rapidity distributions of the different (strange) particle species. In p-Pb collisions, strangeness is locally (in rapidity) not conserved. The present comparison to the data of the WA97 and NA49 Collaborations clearly supports the suggestion that conventional (free) hadronic scenarios are unable to describe the observed high (anti-)hyperon yields in central collisions. A reduction of the constituent quark masses to the current quark masses m(s) ~ 230 Me V, m(q) ~ 10 MeV, as motivated by chiral symmetry restoration, yields hyperon production close to the experimentally observed high values. An ad hoc overall increase of the color electric field strength (effective string tension of κ = 3 Ge V/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 Elsevier Science B.V.
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