Dilepton production and reaction dynamics in heavy-ion collisions at SIS energies from coarse-grained transport simulations

Abstract

Dilepton invariant-mass spectra for heavy-ion collisions at SIS 18 and BEVALAC energies are calculated using a coarse-grained time evolution from the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) model. The coarse-graining of the microscopic simulations enables to calculate thermal dilepton emission rates by application of in-medium spectral functions from equilibrium quantum-field theoretical calculations. The results show that extremely high baryon chemical potentials dominate the evolution of the created hot and dense fireball. Consequently, a significant modification of the ρ spectral shape becomes visible in the dilepton invariant-mass spectrum, resulting in an enhancement in the low-mass region Mee = 200 to 600 MeV/c 2 . This enhancement, mainly caused by baryonic effects on the ρ spectral shape, can fully describe the experimentally observed excess above the hadronic cocktail contributions in Ar+KCl (E lab = 1.76 AGeV) reactions as measured by the HADES collaboration and also gives a good explanation of the older DLS Ca+Ca (E lab = 1.04 AGeV) data. For the larger Au+Au (E lab = 1.23 AGeV) system, we predict an even stronger excess from our calculations. A systematic comparison of the results for different system sizes from C+C to Au+Au shows that the thermal dilepton yield increases stronger (∝ A 4/3 ) than the hadronic background contributions, which scale with A, due to its sensitivity on the time evolution of the reaction. We stress that the findings of the present work are consistent with our previous coarse-graining results for the NA60 measurements at top SPS energy. We argue that it is possible to describe the dilepton results from SIS 18 up to SPS energies by considering the modifications of the ρ spectral function inside a hot and dense medium within the same model.However, this also means that lepton pairs from all stages of the reaction will reach the detector. Especially for a theoretical description this is a big challenge, as it demands a realistic description of the whole space-time evolution of the heavy-ion reaction and taking the various dilepton sources into account.On the experimental side several groups have undertaken the challenging task to measure dilepton spectra in heavy-ion collisions and thereby constrain the theoretical predictions. At SPS energies the NA60 Collaboration was able to measure the ρ in-medium spectral function for the first time, thanks to the high precision of the measurement [11]. The results were in line with previous CERES results [12] and found an excess in the invariant mass range from 0.2 to 0.6 GeV/c 2 . This excess can be explained by a broadening of the ρ meson inside the hot and dense medium with small mass shifts [13][14][15][16]. At RHIC, these investigations were extended to even higher collision energies with basically the same results except for less dominant baryonic effects and a larger fraction of dileptons stemming from the QGP [17,18].Still more challenging is the interpretation of the dilepton measurements, which were performed in the...

Dilepton production and resonance properties within a new hadronic transport approach in the context of the GSI-HADES experimental data

“…Secondly, the employment of a coarse-graining approach [37] in combination with the just discussed lowmass contributions offers the unique opportunity of a direct comparison (see Sec. III D 2).…”

“…Therefore, a comparison of the decay yield of the vector mesons produced by both approaches is of particular interest. Although results for a coarse-grained UrQMD dilepton production are reported [37,38], such a direct comparison has not been performed. Additionally, the low-mass contributions are neglected for the UrQMD transport calculations, making a comparison to the corresponding coarse-grained results in this mass region unfeasible.…”

Dileptons and Chiral Symmetry Restoration

“…Of course, one should keep in mind that the chiral condensate at T pc is still at about half of its vacuum value, which, again is in the vicinity of recent estimates in cold nuclear matter at twice nuclear saturation density [13]. Figure 1: Dilepton spectra in heavy-ion collisions from HADES at SIS [3] (left), CERES [5] and NA60 [6] at SPS (middle panels) and STAR [7] at RHIC (right panel), compared to theoretical calculations with QGP and hadronic emission using the same model for the in-medium ρ spectral function [9,10,11,12].…”

Energy, centrality, and momentum dependence of dielectron production at collider energies in a coarse-grained transport approach

“…This description is kept concise here, as the same model was in detail presented previously; for details we refer the reader to references [30,31].…”

“…[29] and has been successfully applied to describe spectra of electromagnetic probes at SIS 18, FAIR and SPS energies [30][31][32][33]. It offers a compromise between the microscopic and macroscopic description of the collision evolution.…”