Observation of enhanced subthresholdK+production in central collisions between heavy nuclei

In the very heavy collision system ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$ ${+}^{197}$Au the ${\mathit{K}}^{+}$ production process was studied as a function of impact parameter at 1 GeV/nucleon, a beam energy well below the free N-N threshold. The ${\mathit{K}}^{+}$ multiplicity increases more than linearly with the number of participant nucleons and the ${\mathit{K}}^{+}$/${\mathrm{\pi }}^{+}$ ratio rises significantly when going from peripheral to central collisions. The measured ${\mathit{K}}^{+}$ double differential cross section is enhanced by a factor of 6 compared to microscopic transport calculations if secondary processes (ΔN→KΛN and ΔΔ→KΛN) are ignored.