Virtual photons (γ*) are an excellent tool to investigate strong-interaction matter under extreme conditions. Their penetrating nature not only enables the study of bulk properties of such a medium (e.g. temperature measurements unbiased by the collective expansion of the fireball) but also an insight to the microscopic structure of matter. The HADES experiment has measured virtual photons in their di-electron decay channel in heavy-ion collisions as well as in proton- and pion-induced reactions in the center-of-mass energy range of a few GeV. In this contribution, we emphasize multi-differential spectra and collective behavior of di-lepton sources in Au+Au collisions at √sNN = 2.42 GeV and Ag+Ag collisions at √sNN = 2.55 and 2.42 GeV and contrast them with hadronic probes at the freeze-out stage.
Niklas Schild (2026) studied this question.