  An enhanced production of strange particles per pion in heavy ion collisions
compared to elementary p+p interactions was observed in Pb+Pb reactions at the
CERN SPS. New results obtained on small colliding systems, C+C and Si+Si, also
show an enhancement. It increases steeply with system size and reaches in the
larger system almost the level measured in Pb+Pb reactions. The shapes of the
transverse mass spectra of strange mesons (K+, K-, phi) and baryons (Lambda,
Antilambda) are consistent with the presence of collective transverse flow,
which is already visible in small systems (C+C) and increases with the system
size. The Antilambda/Lambda ratio together with the K+/K- ratio yields
information about the baryochemical potential mu_B. The potential, determined
at midrapidity, increases with the system size since the stronger stopping
shifts baryons from near beam and projectile rapidity towards midrapidity. For
the 4pi yields mu_B is almost independent of the size of the colliding system.

Nuclear Experiment