System size dependence of strangeness production at 158 GeV/n

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