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Subthreshold antiproton production in nucleus-nucleus collisions.

Antiproton production from the secondary process pp~ 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 p 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 and has a similar magnitude to that of the data. Recently, experiments' were carried out at the Bevalac to detect the antiproton from heavy-ion collisions at energies which are below the threshold for its production from the nucleon-nucleon reaction in the free space. The motivation for such experiments is to look for effects of dense nuclear matter formed in the early stage of the collision on antiproton production so that one can learn about the properties of such a matter. In particular, antiproton production might be a possible probe of the structure of the Dirac sea in the presence of dense nuclear matter. The measured antiproton to negative pion ratio for the collision of two Si nuclei at an incident energy of 2. 1 GeV/nucleon has a surprisingly large value of about 4.3x10 . This is 3 orders of magnitude larger than that predicted by a calculation' incorporating internal motion of the nucleons in the colliding nuclei and is actually comparable to that from assuming that both the antiproton and the pion abundance are given by their equilibrium values. But this is not expected for antiproton due to its small production probability from the nucleon-nucleon reaction. However, a recent study by one of us indicates that antiproton production from the secondary process pp pP is appreciable if we assume that the p meson abundance is given by its equilibrium value. This process is important because of its appreciable cross section and the higher p abundance than other heavy mesons. Assuming that antiproton production occurs at the freezeout density of half the normal nuclear matter density and lasts a time limited by the lifetime of the p meson, i.e., 1.8 fm/c, we have found that the ratio of the antiproton number to that of the negative pion is = 3.3 x 10 for the reaction studied at the Bevalac and is very close to the measured value. But our assumption that the p abundance is given by the equilibrium value has been questioned in Ref. 1 as the p to tr ratio in our model is about 2.6 X 10 2 and is about a factor of 4 larger than that estimated from electron-pair production in Ca+Ca collisions at the same energy. However, the limit set in Ref. 4 is unreliable due to the very large error associated with the dilepton data. Still, it is of interest to find out whether the p meson remains to play an important role in antiproton production if the p meson abundance is dynamically determined in the collision. In the following, we shall report the results from such an improved study.