System-Size Dependence of Strangeness Production in Nucleus-Nucleus Collisions at $\sqrt{s_{NN}}=17.3\mathrm{G}\mathrm{e}\mathrm{V}$

Emission of ${\pi }^{\pm }$, $K^{\pm }$, $\varphi$, and $\Lambda$ was measured in near-central $\mathrm{C}+\mathrm{C}$ and $\mathrm{S}\mathrm{i}+\mathrm{S}\mathrm{i}$ collisions at $158A\mathrm{G}\mathrm{e}\mathrm{V}$ beam energy. Together with earlier data for $p+p$, $\mathrm{S}+\mathrm{S}$, and $\mathrm{P}\mathrm{b}+\mathrm{P}\mathrm{b}$, the system-size dependence of relative strangeness production in nucleus-nucleus collisions is obtained. Its fast rise and the saturation observed at about 60 participating nucleons can be understood as the onset of the formation of coherent systems of increasing size.

Figure 1

Transverse mass distributions for $\pi$ and $K$ in $0.1<y<0.33$, for $\varphi$ averaged over $0<y<1.8$, and for $\Lambda$ in $|y|<0.4$ measured in $\mathrm{C}+\mathrm{C}$ (●) and $\mathrm{S}\mathrm{i}+\mathrm{S}\mathrm{i}$ (▲) collisions. Only statistical errors are shown.

Figure 2

Rapidity distributions for $\mathrm{C}+\mathrm{C}$ (●) and $\mathrm{S}\mathrm{i}+\mathrm{S}\mathrm{i}$ (▲) collisions. Open symbols are reflected at midrapidity. Error bars are statistical; systematic errors at midrapidity are indicated by the rectangular boxes. The curves represent fits to the data as described in the text.

Figure 3

Experimental ratios of $⟨K^{+}⟩$, $⟨K^{-}⟩$, $⟨\varphi ⟩$, and $⟨\Lambda ⟩$ to $⟨{\pi }^{\pm }⟩$ plotted as a function of system-size (▼ $p+p$; ● $\mathrm{C}+\mathrm{C}$ and $\mathrm{S}\mathrm{i}+\mathrm{S}\mathrm{i}$; △ $\mathrm{S}+\mathrm{S}$; ■ $\mathrm{P}\mathrm{b}+\mathrm{P}\mathrm{b}$). Statistical errors are shown as error bars, systematic errors if available as rectangular boxes. The curves are shown to guide the eye and represent a functional form $a-b·\exp (-⟨N_{\mathrm{p}\mathrm{a}\mathrm{r}\mathrm{t}}⟩/40)$. At $⟨N_{\mathrm{p}\mathrm{a}\mathrm{r}\mathrm{t}}⟩=60$ they rise to about 80% of the difference of the ratios between $N_{\mathrm{p}\mathrm{a}\mathrm{r}\mathrm{t}}=2$ and 400.