Subthreshold Xi- Production in Collisions of p(3.5 GeV)+Nb

Results on the production of the double-strange cascade hyperon ${\mathrm{\Xi }}^{-}$ are reported for collisions of p,(3.5~GeV),+,Nb, studied with the High Acceptance Di-Electron Spectrometer (HADES) at SIS18 at GSI Helmholtzzentrum for Heavy-Ion Research, Darmstadt. For the first time, subthreshold ${\mathrm{\Xi }}^{-}$ production is observed in proton-nucleus interactions. Assuming a ${\mathrm{\Xi }}^{-}$ phase-space distribution similar to that of $\mathrm{\Lambda }$ hyperons, the production probability amounts to $P_{${\mathrm{\Xi }}^{-}$}=(2.0,\pm0.4,$(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})$,\pm 0.3,$(\mathrm{n}\mathrm{o}\mathrm{r}\mathrm{m})$,\pm 0.6,$(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})$)\times10^{-4}$resultingina$${\mathrm{\Xi }}^{-}\mathrm{/}(\mathrm{\Lambda }+{\mathrm{\Sigma }}^0)$$ratioof$P_{${\mathrm{\Xi }}^{-}$}/
P_{$\mathrm{\Lambda }+{\mathrm{\Sigma }}^0$}=(1.2\pm 0.3,$(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})$\pm0.4,$(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})$)\times10^{-2}$. Available model predictions are significantly lower than the estimated ${\mathrm{\Xi }}^{-}$ yield.