The centrality dependence of (multi-)strange hadron abundances is studied for
Pb(158 AGeV)Pb reactions and compared to p(158 GeV)Pb collisions. The
microscopic transport model UrQMD is used for this analysis. The predicted
Lambda/pi-, Xi-/pi- and Omega-/pi- ratios are enhanced due to rescattering in
central Pb-Pb collisions as compared to peripheral Pb-Pb or p-Pb collisions. A
reduction of the constituent quark masses to the current quark masses m_s \sim
230 MeV, m_q \sim 10 MeV, as motivated by chiral symmetry restoration, enhances
the hyperon yields to the experimentally observed high values. Similar results
are obtained by an ad hoc overall increase of the color electric field strength
(effective string tension of kappa=3 GeV/fm). The enhancement depends strongly
on the kinematical cuts. The maximum enhancement is predicted around
midrapidity. For Lambda's, strangeness suppression is predicted at
projectile/target rapidity. For Omega'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. The doubling of the strangeness to nonstrange
suppression factor, gamma_s \approx 0.65, might be interpreted as a signal of a
phase of nearly massless particles.Comment: published version, discussion on strange mesons and new table added,
extended discussion on strange baryon yields. Latex, 20 pages, including 5
eps-figure
