Charged kaon and pion production at midrapidity in proton-nucleus and sulphur-nucleus collisions

The NA44 Collaboration has measured charged kaon and pion distributions at midrapidity in sulphur and proton collisions with nuclear targets at 200 and $450\hspace{0.5em}\mathrm{GeV}/c$ 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 $\mathrm{S}A$ 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 $\overline{p}/p$ ratio. The ${\pi }^{-}/{\pi }^{+}$ ratio is close to unity for all systems. From $p\mathrm{Be}$ to $\mathrm{SPb}$ the $K^{+}/p$ ratio decreases while $K^{-}/\overline{p}$ increases and $\sqrt{{(K}^{+}\cdot K^{-})/(p\cdot \overline{p})}$ stays constant. These data suggest that as larger nuclei collide, the resulting system has a larger transverse expansion and baryon density and an increasing fraction of strange quarks.