Dilepton Production in Heavy Ion Collisions

Abstract

Nucleus-nucleus collisions provide probes of dense and hot nuclear matter far from the groundstate. One of the major aims is to obtain information about the nuclear equation of state and to extract signals which can directly be related to the high compression zone of the system. Hadronic probes often suffer from strong final state interactions, such that the primary information carried by these particles might be washed out due to rescattering in the expansion phase. Alternatively, electromagnetic probes are expected to provide a more unique signal since they leave the hadronic environment after their production without sizable disturbances. In fact, the observation of hard photons from intermediate-energy heavy-ion collisions has led to widespread activities, both on the experimental^1’2 and theoretical side^3’4. Since the use of real photons is restricted to bombarding energies below about 300 MeV/u due to the experimental difficulty to distinguish the direct photons from the decay photons of the copiously produced π⁰’s ², it is natural to use at higher energies virtual photons as they manifest themselves in the production of dileptons. After many years of difficult experimental developments, the Berkeley group has recently obtained first results on dilepton production at BEVALAC energies⁵. It is therefore a natural challenge to see how well these yields can be understood theoretically on the same basis as the hard real photons.

Work supported by BMFT and GSI Darmstadt.

Supported by the German Academic Exchange Service (DAAD) and by the CONICET of Argentina.