Speaker
Beatrice Ramstein
(Institut de Physique Nucleaire)
Description
Originally designed to study medium effects in $e^+e^-$ production in heavy-ion reactions in the SIS-18 energy range (1-3 GeV/nucleon) [1] installed at GSI is a versatile detector. Its excellent particle identification capabilities allowed for a systematic investigations of dielectron, strange particles and pion production in proton, deuteron or heavy-ion induced reactions on proton or nucleus. The obtained dilepton spectra measured at various beam energies show important contributions from baryon resonances decays ($R\to Ne^+e^-$) and a strong influence of the intermediate vector mesons ($\rho / \omega / \phi$) in the corresponding time-like electromagnetic form factors.
In order to directly access such transitions, HADES has started a dedicated pion-nucleon programme [2].
For the first time, combined measurements of hadronic and dielectron final states have been performed in $\pi$-N reactions, using polyethylene and carbon targets.
Differential cross-sections of the exclusive channels with two pions in the final state ($\pi^-\pi^+ n$, $\pi^0\pi^- p$) were obtained in the second resonance region with an unprecedented statistics. These new data were included in the partial wave analysis (PWA) of the Bonn-Gatchina group [3] together
with the world data on pion and photon production. The obtained solution provides
the excitation function of the two-pion production in photo- and pion- induced reactions around the pole of the N(1520)D$_{13}$ resonance and of its decomposition into the different resonant, non-resonant and $\rho$ contributions.
Results for the exclusive $ne^+e^-$ production will be compared to various model calculations.
In addition, the results about the $\rho$ contribution in the two-pion production channels are used to investigate the validity of the Vector Dominance Model for baryon transitions. The angular distributions of the leptons, which contain additional information on the electromagnetic structure of the different transitions, will be discussed.
Finally, the prospects for HADES measurements at SIS-18 in the near future and later at SIS-100 (FAIR) will be presented.
[1] G. Agakishiev et al. (HADES), Eur. Phys. J. A 41 (2009) 243.
[2] P. Salabura, J. Stroth, L. Fabbietti (HADES), Nucl. Phys. News 25 (2015) 22.
[3] A.V. Anisovich, E. Klempt, A.V. Sarantsev, U. Thoma, Eur. Phys. J. A 24 (2005) 111.
Primary author
Beatrice Ramstein
(Institut de Physique Nucleaire)
