Abstract:
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.