Results on the production of the double-strange cascade hyperon
Ξ− 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 Ξ− production is observed in proton-nucleus
interactions. Assuming a Ξ− phase-space distribution similar to
that of Λ hyperons, the production probability amounts to
PΞ−=(2.0±0.4(stat)±0.3(norm)±0.6(syst))×10−4 resulting in a
Ξ−/(Λ+Σ0) ratio of $P_{\mathrm{\Xi^-}}/\
P_{\mathrm{\Lambda+\Sigma^0}}=(1.2\pm
0.3\,\mathrm{(stat)}\pm0.4\,\mathrm{(syst)})\times10^{-2}.Availablemodelpredictionsaresignificantlylowerthantheestimated\mathrm{\Xi^-}$ yield.Comment: 6 pages, 3 figure