Characterising the hot and dense fireball with virtual photons at HADES

Abstract

Electromagnetic probes ($\gamma, \gamma^{\ast}$) offer a unique opportunity to study the conditions in heavy-ion collisions throughout their whole evolution. Once created, these particles can travel largely unhindered through the strongly interacting medium and bring direct information from their origins to a detector. Virtual photons, decaying into lepton pairs, serve as particularly interesting because they also carry additional information encoded in their invariant mass. In this contribution, work-in-progress measurements of such dileptons are presented. Based on high statistics experiments of Au+Au and Ag+Ag collisions, collected with the High-Acceptance-DiElectron-Spectrometer (HADES), at center-of-mass energy of $\sqrt{s_{NN}}=2.42$ GeV and $\sqrt{s_{NN}}=2.55$ GeV respectively, various dilepton observables can be extracted. This includes the invariant mass spectra, allowing a determination of the in-medium spectral functions, fireball temperature as well as the overall dilepton yield, which can be connected to the lifetime of the fireball. Furthermore, first work-in-progress results on the dilepton anisotropic flow coefficients are presented, giving more insights into the collective properties of the hot and dense medium.