Subthreshold antiproton and K^- production in heavy ion collisions

Subthreshold -p and K^- and energetic π^- production was studied in Ne + NaF, Cu, Sn and Bi, and in Ni + Ni collisions with incident energies between 1.6 and 2 GeV/u. The measured cross sections indicate a dominant contribution of baryonic resonances. This is also consistent with a generalized scaling behaviour of the cross sections with the energy available in the collision and the energy necessary to produce particles as observed with Ne induced reactions. Deviations from scaling especially pronounced in the Ni-Ni system will be discussed in terms of absorption effects. The flat slope of the excitation function for -p production can be related to a reduced production threshold caused by a reduction of the antiproton mass in the dense and heated medium by about 100-150 MeV/c². A similar in-medium mass reduction is also indicated for K^- mesons. An increased -p reabsorption probability for the heavier systems is concluded from the comparison of the -p yields in Ne + NaF, Ne + Sn and Ni + Ni collisions.