Abstract
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.
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