Measurement of "subthreshold" Production Ofk−and Antiprotons in Relativistic Nuclear Collisions

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