Dilepton production in intermediate energy nucleus-nucleus collisions as well as in elementary proton-proton reactions is analysed within the UrQMD transport model. For C+C collisions at 1 AGeV and 2 AGeV the resulting invariant mass spectra are compared to recent HADES data. We find that the experimental spectrum for C+C at 2 AGeV is slightly overestimated by the theoretical calculations in the region around the vector meson peak, but fairly described in the low mass region, where the data is satisfactorily saturated by the Dalitz decay of the η meson and the ∆ resonance. At 1 AGeV an underestimation of the experimental data is found, pointing that at lower energies the low mass region is not fully saturated by standardly parametrized ∆ Dalitz decays alone. Furthermore, predictions for dilepton spectra for pp reactions at 1.25 GeV, 2.2 GeV and 3.5 GeV and Ar+KCl reactions at 1.75 AGeV are presented. The study is complemented by a detailed investigation of the role of absorption of the parent particles on the corresponding dilepton yields in the regime which has so far been probed by HADES.

The in-medium spectral functions of ρ and ω mesons and the broadening of nucleon resonances at finite baryon density are calculated self-consistently by combining a resonance dominance model for the vector meson production with an extended vector meson dominance model. The influence of the in-medium modifications of the vector meson properties on the dilepton spectrum in heavy-ion collisions is investigated. The dilepton spectrum is generated for the C+C reaction at 2.0A GeV and compared with recent HADES Collaboration data. The collision dynamics is then described by the Tübingen relativistic quantum molecular dynamics transport model. We find that an iterative calculation of the vector meson spectral functions that takes into account the broadening of the nucleon resonances due to their increased in-medium decay branchings is convergent and provides a reasonable description of the experimental data in the mass region 0.45 ≤ M ≤ 0.75 GeV. On the other side, the theoretical calculations slightly underestimate the region m π ≤ M ≤ 0.4 GeV. Popular in-medium scenarios such as a schematic collisional broadening and dropping vector mesons masses are discussed as well.

Dileptons represent a unique probe for nuclear matter under extreme conditions reached in heavy-ion collisions. They allow to study meson properties, like mass and decay width, at various density and temperature regimes. Present days models allow generally a good description of dilepton spectra in ultra-relativistic heavy ion collision. For the energy regime of a few GeV/nucleon, important discrepancies between theory and experiment, known as the DLS puzzle, have been observed. Various models, including the one developed by the T\"{u}bingen group, have tried to address this problem, but have proven only partially successful. High precision spectra of dilepton emission in heavy-ion reactions at 1 and 2 GeV/nucleon will be released in the near future by the HADES Collaboration at GSI. Here we present the predictions for dilepton spectra in C+C reactions at 1 and 2 GeV/nucleon and investigate up to what degree possible scenarios for the in-medium modification of vector mesons properties are accessible by the HADES experiment.Comment: 12 pages, 4 figures; submitted to Phys.Lett.

We show that the measurement of di-leptons might provide only a restricted view into the most dense stages of heavy ion reactions. Thus, possible studies of meson and baryon properties at high baryon densities, as e.g. done at GSI-HADES and envisioned for FAIR-CBM, might observe weaker effects than currently expected in certain approaches. We argue that the strong absorption of resonances in the high baryon density region of the heavy ion collision masks information from the early hot and dense phase due to a strong increase of the total decay width because of collisional broadening. To obtain additional information, we also compare the currently used approaches to extract di-leptons from transport simulations -i.e. shining, only vector mesons from final baryon resonance decays and instant emission of di-leptons and find a strong sensitivity on the method employed in particular at FAIR and SPS energies. It is shown explicitly that a restriction to ρ meson (and therefore di-lepton) production only in final state baryon resonance decays provide a strong bias towards rather low baryon densities. The results presented are obtained from UrQMD v2.3 calculations using the standard set-up. Quantum-Chromo-Dynamics (QCD) predicts that the properties of hadrons change when they are brought into a (hot and/or dense) nuclear environment [1,2]. This modification is due to the interaction with the surrounding medium which eventually leads to chiral symmetry restoration at high baryon densities and/or high temperatures [3]. The experimental verification of this theoretical prediction is one of the most challenging questions in modern strongly interacting matter physics.Among the non-strange mesons the ρ meson plays a dominant role in these investigations. It has a short lifetime and therefore it has a large probability to decay inside the reaction zone when created in heavy ion collisions. It couples strongly to nuclear resonances and, most important, it has a non-negligible chance to decay into dileptons which leave the interaction zone essentially without any further interaction. Thus, the di-lepton channel seems to offer a unique chance to study the high baryon density properties of the ρ meson.Theoretically the question of how the spectral function of the ρ meson changes in the medium is still under active discussion. There is certain theoretical evidence that the ρ meson is broadened if put into the nuclear medium [4,5,6]. In contrast, Hatsuda and Lee predicted a lowering of the ρ meson mass in a nuclear environment based on QCD sum rules calculations [1]. A result which has also been found by Brown and Rho [2,7,8]. On the other hand, more recent calculations indicate that the pole mass of the ρ meson remains almost unchanged in the nuclear medium [9,10,11]. However, these calculations rely on specific assumptions on the coupling strength of the ρ meson to the nuclear resonances and on the branching ratios whose validity can presently only be proven by comparison to experimental data. For the present status of the theoreti...

We analyze dilepton emission from hot and dense matter using a hybrid approach based on the Ultrarelativistic Quantum Molecular Dynamics (UrQMD) transport model with an intermediate hydrodynamic stage for the description of heavy-ion collisions at relativistic energies. During the hydrodynamic stage, the production of lepton pairs is described by radiation rates for a strongly interacting medium in thermal equilibrium. In the low mass region, hadronic thermal emission is evaluated assuming vector meson dominance including in-medium modifications of the ρ meson spectral function through scattering from nucleons and pions in the heat bath. In the intermediate mass region, the hadronic rate is essentially determined by multi-pion annihilation processes. Emission from quark-antiquark annihilation in the quark gluon plasma is taken into account as well.When the system is sufficiently dilute, the hydrodynamic description breaks down and a transition to a final cascade stage is performed. In this stage dimuon emission is evaluated as commonly done in transport models. Focusing on the enhancement with respect to the contribution from long-lived hadron decays after freezout observed at the SPS in the low mass region of the dilepton spectra, the relative importance of the different thermal contributions and of the two dynamical stages is investigated. We find that three separated regions can be identified in the invariant mass spectra. Whereas the very low and the intermediate mass regions mostly receive contribution from the thermal dilepton emission, the region around the vector meson peak is dominated by the cascade emission. Above the ρ-peak region the spectrum is driven by QGP radiation. Analysis of the dimuon transverse mass spectra reveals that the thermal hadronic emission shows an evident mass ordering not present in the emission from the QGP. A comparison of our calculation to recent acceptance corrected NA60 data on invariant as well as transverse mass spectra is performed.