Subthreshold antiproton production in nucleus-nucleus collisions

Antiproton production from the secondary process ρρ→pp¯ is studied for nucleus-nucleus collisions at energies which are below the threshold for its production from the nucleon-nucleon reaction in the free space. In an expanding fireball model, we determine the ρ abundance from the fusion of pions and find that for collisions at 2.1 GeV/nucleon the antiproton to negative pion ratio is ≊5.5×${10}^{\mathrm{-}7}$ and has a similar magnitude to that of the data.