DILEPTON PRODUCTION IN HEAVY-ION COLLISIONS

The dynamical evolution of nucleus-nucleus collisions is described by a transport equation of the Uehling-Uhlenbeck type. Our model evolves phase-space distribution functions for nucleons, Δ's, N*(1440)-resonances and pions with their isospin degrees of freedom. The equations are solved by the test-particle simulation method. We apply this model to proton-nucleus and nucleus-nucleus collisions from 400 MeV/A to 2.1 GeV/A. Our approach is found to correctly describe the spectra of baryons and pions. Furthermore, we calculate the production of dileptons considering proton-neutron and pion bremsstrahlung, Δ and N* decay and π+π- annihilation. The calculations reproduce the data for p+9Be and for 40Ca + 40Ca at the bombarding energies of 1 GeV/A and 2 GeV/A except for small invariant masses (M≤ 200 MeV) where the situation is not fully understood. Above 300 MeV dilepton invariant mass the relevant contributions arise from proton-neutron bremsstrahlung, the Δ Dalitz-decay and π+ π- annihilation. At 2 GeV/A for 40Ca + 40Ca, however, we find the bremsstrahlung contribution to be higher than that from pion annihilation. © 1990.