What happens when gold nuclei, accelerated to about 90% of the speed of light, strike gold nuclei at rest? For an extremely short time, t~10
Virtual photons, the generalized form of electromagnetic radiation, materialize after short time by formation of a pair of charged leptons, e.g. an electron and a positron. Throughout the course of a heavy-ion collision such photons offer the unique opportunity to directly monitor “Roentgen-images” (in-medium electromagnetic spectral functions) and to measure “Planck-like-spectra” (temperature of the emitting source) of strongly interacting matter.
This talk will discuss important experimental results on emissivity of matter obtained by the High Acceptance DiElectron Spectrometer at heavy-ion synchrotron SIS18, GSI-Darmstadt. A deeper understanding of the microscopic origin of the excess radiation requires systematic investigation of di-electron radiation emitted from baryonic resonances produced off protons in pion-induced reactions. These are studied in HADES at GSI making use of pion beams.