Thermal Dileptons from Coarse-Grained Transport as Fireball Probes at SIS Energies

TL;DR

This study uses a coarse-graining approach to analyze thermal dilepton emission in Au+Au collisions at SIS energies, linking electromagnetic radiation to fireball lifetime, temperature, and collective effects, and validating the method across energy regimes.

Contribution

It introduces a novel coarse-graining method to connect hadronic transport simulations with thermal dilepton emission, providing insights into fireball properties at SIS energies.

Findings

Dilepton radiation begins after 7 fm/c and lasts for about 13 fm/c.

The fireball lifetime is smaller than naive density evolution estimates.

Dilepton yields correlate with fireball lifetime and temperature, serving as effective probes.

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 $\rho$ 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 7 fm/c, and lasts for about 13 fm/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 20-30 fm/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 $M=0.3-0.7$ GeV/$c^{2}$, 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 $\phi$ 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.