Dilepton production and mT-scaling at BEVALAC/SIS energies

Abstract

We present a dynamical study of e + e − production in C + C and Ca + Ca collisions at BEVALAC/SIS energies on the basis of the covariant transport approach HSD employing momentum-dependent ρ-meson spectral functions that include the pion modifications in the nuclear medium as well as the polarization of the ρ-meson due to resonant ρ−N scattering. We find that the experimental data from the DLS collaboration cannot be described within the ρ-meson spectral function approach. A dropping η-mass scenario leads to a good reproduction of the DLS dilepton data, however, violates the m Tscaling of π 0 and η spectra as observed by the TAPS collaboration as well as η photoproduction on nuclei. PACS

Role of baryonic resonances in the dilepton emission in nucleon-nucleon collisions

“…Enhancements (above known sources) observed in the measured [15,16] soft lepton pair production cross sections in heavy ion collisions at the SPS energies, have been attributed [17][18][19][20][21] to in-medium modifications of the vector meson properties rather than to reflection of a new state of hadronic matter. Although, such a scenario has not been fully successful [22][23][24] in explaining the dilepton spectra measured in these reactions at much lower beam energies (a few GeV/nucleon) [25] where the temperature and density regime is different and one does not expect a phase transition.…”

Probing dense and hot matter with low-mass di-leptons and photons

“…5 compares the data with the same cocktail using this time the ρ meson spectral function modified in the medium. Part of the enhancement is explained but the calculations are still a factor of 2-3 below the data [20]. Over the last few years, the HADES experiment at the GSI has collected data on di-electron production in C+C collisions at 1 and 2 AGeV and first results should be available soon.…”

“…5. In the left panel, the results are compared to a cocktail including hadrons decays, proton-neutron (pn) and pion-nucleon (πN) bremsstrahlung and also the pion annihilation (π + π − → ρ → e + e − ) using the vacuum or free ρ spectral function [20]. An enhancement of low-mass pairs is observed in the mass range m = 0.2 -0.6 GeV/c 2 which looks qualitatively similar to the CERES results.…”

Analysis of the exclusive final state npe+e- in the quasi-free np reaction

“…The pp data are well described by calculations with effective Lagrangian models, except [4,5] which overestimates the measured yields. Various transport models [13][14][15], adding incoherently contributions from Δ Dalitz decay and from p-p bremsstrahlung (calculated in the soft photon approximation) describe the data well. The dominant contribution is the Δ Dalitz decay with the dielectron invariant mass distribution slightly depending on the choice of the corresponding transition form-factors [16,17].…”