Abstract
A dedicated programme aiming at studying electromagnetic baryonic transitions in the time-like region has started at GSI using the pion beam and the High Acceptance Di-Electron Spectrometer (HADES) set-up. A pioneering experiment has been carried out in the second resonance region, at a center-of-mass energy of 1.49 GeV, using carbon and polyethylene targets, allowing for an analysis of the inclusivee+e-production and of the exclusive quasi-freeπ-p→ne+e-reaction, in complement to hadronic channels. Predictions for the Dalitz decay of N(1520) and N(1535) based on real photon couplings strongly underestimate thee+e-yield at invariant masses larger than 300 MeV/c2, which signals effects of time-like baryon transition form factors, in qualitative agreement with the Vector Dominance Models (VDM). A quantitative description of the observede+e-yield is achieved by taking into account the contribution from off-shellρs. The latter was provided by the preliminary results of a Bonn-Gatchina Partial Wave Analysis of the two-pion production channels, which were measured simultaneously in our experiment. A good agreement is also obtained using a covariant model for the time-like electromagnetic form factors for the N-N(1520) transition. The angular distributions for thee+e-production contain additional selective information on the spin structure of the different transitions. The measurements with the pion beam will be extended in future in the third resonance region. Hyperon Dalitz decay studies in proton induced reactions are also foreseen.
Highlights
Dilepton production is a favored probe for QCD matter studies
The main objective of experiments with the High Acceptance Di-Electron Spectrometer (HADES) [2] at GSI is the study of hadronic matter in the low temperature (T< 80 MeV) and large density region [3]
Salabura’s report at this conference [4] and preliminary results concerning the hadronic channels measured in π− p collisions in the second resonance region can be found in the contribution of I
Summary
The emission of low mass dileptons gives insight on the properties of vector mesons in the hadronic matter It allows to search for in-medium modifications of the ρ mesons [1], as predicted by hadronic many-body calculations [1] due to the dressing of the pions and the direct coupling of the ρ meson to baryons. Salabura’s report at this conference [4] and preliminary results concerning the hadronic channels measured in π− p collisions in the second resonance region can be found in the contribution of I. Ciepał to this conference [5].
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