Probing dense baryon-rich matter with virtual photons

Quantum chromodynamics (QCD) supports the existence of a novel state of strongly interacting matter with temperatures above kT = 50 MeV and densities several times higher than those in nuclei. Such matter can be created in the laboratory as a transient state by colliding heavy ions at relativistic energies. The thermal electromagnetic radiation emitted from the dense stage of these collisions has been used to probe microscopic properties of the produced medium. The spectral distribution of virtual photons – manifesting themselves as di-electron pairs – shows a nearly exponential shape which indeed points to an emitting source of temperature in excess of 70 MeV/k and to a strong modification of the properties of its constituents. Regarding bulk properties, this medium is similar to the dense matter formed in the final state of a neutron star merger, as apparent from its recent multi-messenger observation.