Understanding the properties of the fireball with the polarization signature of thermal dileptons

¹ Technische Universität Darmstadt, 64289 Darmstadt, Germany
² GSI Helmholtzzentrum für Schwerionenforschung GmbH, 64291 Darmstadt, Germany
³ Institut für Theoretische Physik, Goethe-Universität Frankfurt, 60438 Frankfurt (Main), Germany
⁴ Helmholtz Research Academy Hesse for FAIR, Campus Frankfurt, 60438 Frankfurt (Main), Germany
⁵ Theoretical Physics Department, CERN, 1211 Geneva 23, Switzerland
⁶ Texas A&M University, College Station, TX 77843-3366, USA

^* e-mail: f.seck@gsi.de

Published online: 26 June 2024

Abstract

Multi-differential measurements of dilepton spectra serve as a unique tool to characterize the properties of matter in the interior of the hot and dense fireball created in heavy-ion collisions. An important property of virtual photons is their spin polarization defined in the rest frame of the virtual photon with respect to a chosen quantization axis. Microscopic calculations of in-medium electromagnetic spectral functions have mostly focused on integrated yields which are proportional to the sum of the longitudinal and transverse components of the virtual photon’s self-energy, while photon polarization results from the difference of these components. As the processes that drive the medium effects in the spectral function change with invariant mass and momentum, this becomes a powerful tool for studying the medium composition. We present the polarization observables of thermal virtual photons as a function of mass and momentum and confront the results with existing measurements from HADES and NA60.