Subthreshold pion production in nucleus-nucleus collision within the quantum molecular dynamics approach

The subthreshold pion production from 12C+12C, 20Ne+20Ne, 40 Ca+40 Ca, 20Ne+64Cu and 93Nb+93Nb collisoins is studied within the quantum molecular dynamics (QMD) model. For the mean field part of the theory, we use both the soft and the hard equations of state (EOS) with and without the momentum dependence in the interaction, while for the in-medium NN cross section, the Brueckner G-matrix is utilized. The pion yield from a nucleus-nucleus collision is assumed to be the incoherent sum of the corresponding yields from the individual nucleon-nucleon collisions. For the latter process, parametrized cross sections are adopted. The dependence of the pion cross section on the different nuclear EOS, on the momentum dependence in the interaction and on the pion reabsorption factor is studied. The time sequence of the pion production is also analyzed. The theoretical results for 20Ne+20Ne (Elab/A = 183 MeV) and 20Ne+64Cu (Elab/A = 250 MeV) collisions are compared with available experimental data. (orig.)