SUBTHRESHOLD ANTIPROTON AND K- PRODUCTION IN HEAVY-ION COLLISIONS

Subthreshold pBAR and K- and energetic pi- production was studied in N e + NaF, Cu, Sn and Bi, and in Ni + Ni collisions with incident energi es between 1.6 and 2 GeV/u. The measured cross sections indicate a dom inant contribution of baryonic resonances. This is also consistent wit h a generalized scaling behaviour of the cross sections with the energ y available in the collision and the energy necessary to produce parti cles as observed with Ne induced reactions. Deviations from scaling es pecially pronounced in the Ni - Ni system will be discussed in terms o f absorption effects. The flat slope of the excitation function for pB AR production can be related to a reduced production threshold caused by a reduction of the antiproton mass in the dense and heated medium b y about 100-150 MeV/c2. A similar in-medium mass reduction is also ind icated for K- mesons. An increased pBAR reabsorption probability for t he heavier systems is concluded from the comparison of the pBAR yields in Ne + NaF, Ne + Sn and Ni + Ni collisions.