Eur. Phys. J. A (2016) 52: 131
https://doi.org/10.1140/epja/i2016-16131-1

Regular Article - Theoretical Physics

Thermal dileptons from coarse-grained transport as fireball probes at SIS energies

¹ Technische Universität Darmstadt, 64289, Darmstadt, Germany
² GSI Helmholtzzentrum für Schwerionenforschung GmbH, 64291, Darmstadt, Germany
³ Texas A&M University, 77843-3366, College Station, TX, USA
⁴ Institut für Kernphysik, Goethe-Universität, 60438, Frankfurt, Germany

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

Received: 29 December 2015
Revised: 7 March 2016
Accepted: 23 March 2016
Published online: 16 May 2016

Abstract

Utilizing a coarse-graining method to convert hadronic transport simulations of Au+Au collisions at SIS energies into local temperature, baryon and pion densities, we compute the pertinent radiation of thermal dileptons based on an in-medium spectral function that describes available spectra at ultrarelativistic collision energies. In particular, we analyze how far the resulting yields and slopes of the invariant-mass spectra can probe the lifetime and temperatures of the fireball. We find that dilepton radiation sets in after the initial overlap phase of the colliding nuclei of about 7fm/c, and lasts for about 13fm/c. This duration closely coincides with the development of the transverse collectivity of the baryons, thus establishing a direct correlation between hadronic collective effects and thermal EM radiation, and supporting a near local equilibration of the system. This fireball “lifetime” is substantially smaller than the typical 20-30fm/c that naive considerations of the density evolution alone would suggest. We furthermore find that the total dilepton yield radiated into the invariant-mass window of -0.7GeV/c^2 normalized to the number of charged pions, follows a relation to the lifetime found earlier in the (ultra-) relativistic regime of heavy-ion collisions, and thus corroborates the versatility of this tool. The spectral slopes of the invariant-mass spectra above the -meson mass provide a thermometer of the hottest phases of the collision, and agree well with the maximal temperatures extracted from the coarse-grained hadron spectra.