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Probing cosmic matter in the laboratory with virtual photons
Cosmic forms of matter can be created in the laboratory for short instances in time by colliding heavy nuclei at (ultra-) relativistic energies. One challenge of such studies is to search for exotic phases of strongly interacting matter by isolating unambiguous signals. At very high energies (RHIC, LHC) matter is created as it has been realized in the cosmos a few micro seconds after the Big Bang. At lower beam energies (GSI, FAIR) the matter formed resembles properties as they may exist in the interior of compact stellar objects. Virtual and real photon emission from such exotic forms of matter is a promising probe since they leave the hot and dense interaction zone almost undisturbed. In this way, they carry information of the microscopic properties of matter to the detectors. The key objective of using dileptons as probes is to identify and study modifications of hadrons in the QCD medium, search for the onset of deconfinement and chiral symmetry restoration, and possibly to discover unconventional states of matter.