${\Omega }^{-}$ and ${\overline{\Omega }}^{+}$ Production in Central $\mathrm{Pb}+\mathrm{Pb}$ collisions at 40 and $158A\mathrm{GeV}$

Results are presented on $\Omega$ production in central $\mathrm{Pb}+\mathrm{Pb}$ collisions at 40 and $158A\mathrm{GeV}$ beam energy. For the first time in heavy ion reactions, rapidity distributions and total yields were measured for the sum ${\Omega }^{-}+{\overline{\Omega }}^{+}$ at $40A\mathrm{GeV}$ and for ${\Omega }^{-}$ and ${\overline{\Omega }}^{+}$ separately at $158A\mathrm{GeV}$. The yields are strongly underpredicted by the string-hadronic UrQMD model but agree better with predictions from hadron gas models.

Figure 1

The invariant-mass distributions of ${\Omega }^{-}$ and ${\overline{\Omega }}^{+}$ candidates. Left: summed distribution of $\Lambda K^{-}$ and $\overline{\Lambda }{K}^{+}$ pairs at $40A\mathrm{GeV}$. Middle: $\Lambda K^{-}$ pairs at $158A\mathrm{GeV}$. Right: $\overline{\Lambda }{K}^{+}$ pairs at $158A\mathrm{GeV}$. The solid curves represent a fit to signal and background described in the text. The dashed curves show the background contribution.

Figure 2

The transverse-mass spectra of ${\Omega }^{-}$ and ${\overline{\Omega }}^{+}$ at midrapidity. Left: ${\Omega }^{-}+{\overline{\Omega }}^{+}$ at $40A\mathrm{GeV}$ (triangles) and ${\Omega }^{-}$ at $158A\mathrm{GeV}$ (circles). Right: ${\overline{\Omega }}^{+}$ at $158A\mathrm{GeV}$. The errors shown are statistical only. The dashed curve shows the exponential fit described in the text. The solid, dotted, and dash-dotted curves show a model including transverse expansion.

Figure 3

The rapidity dependence of $\Omega$ production in central $\mathrm{Pb}+\mathrm{Pb}$ collisions. Left: ${\Omega }^{-}+{\overline{\Omega }}^{+}$ at $40A\mathrm{GeV}$ (triangles) and ${\Omega }^{-}$ at $158A\mathrm{GeV}$ (circles). Right: ${\overline{\Omega }}^{+}$ at $158A\mathrm{GeV}$. The errors shown are statistical only. The open symbols show the measured points (full symbols) reflected around midrapidity. The curves correspond to Gaussian fits to the data.

Figure 4

The ratio $⟨\Omega ⟩/⟨\pi ⟩$ (see text) versus the center-of-mass energy. Statistical errors are shown as lines, while the brackets denote the systematic errors. The dashed curve shows the prediction from the hadronic string model UrQMD [25] and the gray box that of RQMD [26]. A hadron gas model without strangeness suppression [28] is shown by the solid curve. The open squares represent the fits from [27] including strangeness undersaturation.