Subthreshold antiproton production inSi28collisions at 2.1 GeV/nucleon

Abstract
We report on the first observation of subthreshold antiproton production in nucleus-nucleus collisions. This measurement was made for the system Si28 at a bombarding energy of 2.1 GeV/nucleon (kinetic energy per NN pair in the c.m. frame ∼850 MeV). A differential cross section d2σ/dP d Ω of 80±40 nb/sr (GeV/c) was measured for production at 1.9 GeV/c and 0°. This result is 3 orders of magnitude larger than that predicted by a calculation incorporating internal motion of the nucleons in the colliding nuclei.