Measurement of "Subthreshold" Production ofK−and Antiprotons in Relativistic Nuclear Collisions

The observation of subthreshold production of $K^{-}$ in the reaction ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$+${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ at 2.1 GeV/ nucleon is reported. For production at 0° and 1 GeV/c, a $K^{-}$ cross section ($\frac{d^2\sigma }{\mathrm{dp}d\Omega }$) of 1.6 mb ${\mathrm{sr}}^{-1\mathrm{}}$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^{-1\mathrm{}}$ and an upper limit for $\overline{p}$ production of 3 nb ${\mathrm{sr}}^{-1\mathrm{}}$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^{-1\mathrm{}}$ were measured. Calculations incorporating the effect of nuclear Fermi momentum underpredict the $K^{-}$ yield by more than a factor of 20. Production mechanisms involving collective effects and equilibration are discussed.