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Dilepton production in neutron-proton collisions at HADES energies

Experiments with High Acceptance Di-electron Spectrometer (HADES) are aimed at searching for medium modifications of hadrons at moderately high temperatures (T < 100 MeV) and baryonic densities up to 3 times the normal nuclear matter density. Due to negligible final state interactions with surrounding medium, dileptons provide a very clean and powerful probe for this purpose. Dilepton production was studied by the DLS collaboration at BEVALAC [1] at beam energies of 1–2 GeV/nucleon. A large dilepton excess over the ”hadronic cocktail” was observed in the C + C andCa +Ca collisions which could not be explained by theoretical models. This situation became known as the ”DLS puzzle”. To resolve this issue, the HADES experiment [2] has reinvestigated this reaction inC + C collision at similar beam energies. The remarkable fact is that the HADES data agree well with those of the DLS Collaboration. Therefore, there is no longer any question against the validity of the DLS data and the puzzle has now shifted to the theory side. In a recent transport model calculation it has been shown [3] that if larger cross sections are used for the proton-neutron ( pn) bremsstrahlung process then the excess dilepton yields observed in C + C collisions by both the DLS and the HADES experiments can be reproduced. Enhanced pn bremsstrahlung cross sections, however, are not in agreement with the results presented in Refs. [4] and [5] where a fully relativistic and gaugeinvariant model has been employed to calculate the dilepton production in nucleon-nucleon ( NN ) collisions. On the other hand, calculations reported in Refs. [6] do support a larger value for this cross section. This is somewhat puzzling as the later model uses similar ingredients and the input parameters as those in Refs. [4, 5]. We have taken up a systematic investigation of the dilepton production in theNN collisions at HADES energies within our fully relativistic and gauge-invariant model. W e aim at describing the recent HADES data for this reaction reported very recently in Ref. [7]. Furthermore, we would also like to understand the reason for the differences seen between the results of Refs. [4, 5] and [6] as it has important implications for a proper theoretical interpretation of the HADESC + C data. In this report we examine the role of various meson exchange processes to the dilepton production in the pn collisions. In Fig. 1, we show individual contributions of the πandρ-exchange graphs to the invariant mass distribution of the dilepton production in the pn collisions at 1.25 GeV beam energy. Contributions of other meson exchange processes are relatively smaller. The coherent sum of the two