Strangeness enhancement in heavy ion collisions – evidence for quark-gluon matter?

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 Λʹs, strangeness suppression is predicted at projectile/target rapidity. For Ωʹs, 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.