Dileptons with a coarse-grained transport approach

Abstract: Electromagnetic probes, i.e., lepton-antilepton pairs (dileptons) and photons, are an important probe for the hot and dense matter created in heavy-ion collisions at all reaction energies. Due to their penetrating nature they are the unique messengers from the hot and dense state of the fireball, allowing insight into the medium properties of the electromagnetic current-current correlator during the whole evolution of the collision. In this talk, I shall review the interesting relations of this signal with fundamental properties of strongly interacting matter, particularly the structure of the QCD phase diagram and the chiral phase-transition. Using a coarse-graining approach to the bulk-medium evolution of the fireball, from the early partonic through a hot/dense hadron-resonance gas to the hadronic freeze-out, as described by the UrQMD transport model, makes state-of-the-art thermal quantum-field theoretical calculations for the dilepton-production rates from thermal partonic and hadronic matter available in such realistic bulk-medium descriptions. With this model a good description of the dilepton invariant-mass and p_t and rapidity spectra over a large range of collision energies (from AGS, GSI-SIS energies over CERN-SPS up to BNL-RHIC) is achieved.