  The NA44 collaboration has measured charged kaon and pion distributions at
midrapidity in sulphur and proton collisions with nuclear targets at 200 and
450 GeV/c per nucleon, respectively. The inverse slopes of kaons are larger
than those of pions. The difference in the inverse slopes of pions, kaons and
protons, all measured in our spectrometer, increases with system size and is
consistent with the buildup of collective flow for larger systems. The target
dependence of both the yields and inverse slopes is stronger for the sulphur
beam suggesting the increased importance of secondary rescattering for SA
reactions. The rapidity density, dN/dy, of both K+ and K- increases more
rapidly with system size than for pi+ in a similar rapidity region. This trend
continues with increasing centrality, and according to RQMD, it is caused by
secondary reactions between mesons and baryons. The K-/K+ ratio falls with
increasing system size but more slowly than the pbar/p ratio. The pi-/pi+ ratio
is close to unity for all systems. From pBe to SPb the K+/p ratio decreases
while K-/pbar increases and ({K+*K-}/{p*pbar})**1/2 stays constant. These data
suggest that as larger nuclei collide, the resulting system has a larger
transverse expansion, baryon density and an increasing fraction of strange
quarks.

Nuclear Experiment