Abstract
The High-Acceptance Di-Electron Spectrometer (HADES) operates at the GSI Helmholtzzentrum fur Schwerionenforschung in Darmstadt with pion, proton and heavyion beams provided by the synchrotron SIS18. In summer 2014, HADES took data using a pion beam on carbon and polyethylene targets. A large part of the data was taken at a pion beam momentum of 0.69 GeV/c in order to explore di-pion and di-electron production in the second resonance region and the sub-threshold coupling of the ρ to baryonic resonances. In this contribution lepton identification will be discussed as well the purity of reconstructed e + e − . Finally we will show the preliminary di-electon raw spectra.
Highlights
Previous measurements made by the High-Acceptance Di-Electron Spectrometer (HADES) [1] showed an enhancement in di-lepton spectra below the vector meson pole both in proton-nucleus and in nucleus-nucleus collisions [2] in comparison with a cocktail of known hadronic sources
The Ring Imaging CHerenkov detector (RICH) information was used trying two different approaches: the standard ring finder based on a pattern recognition algorithm to select the good rings [1] and a new approach called backtracking [6], that was developed in order to increase ring identification capabilities taking advantage of tracking information: when a track is found with velocity, momentum and energy loss that correspond to a lepton, it is propagated back to the RICH detector in order to fix the possible ring center on the pad plane
The HADES experiment measured for the first time di-electon production in pion-induced reactions in the second resonance region
Summary
Previous measurements made by the HADES [1] showed an enhancement in di-lepton spectra below the vector meson pole both in proton-nucleus and in nucleus-nucleus collisions [2] in comparison with a cocktail of known hadronic sources. The main interpretation of this effect is based on the strong coupling of the ρ meson to the baryonic resonances (see for example [3]). Exploiting the possibility of GSI to provide a secondary pion beam, HADES has the purpose to study the coupling of the ρ with the baryonic resonances in the second resonance region by means of the reactions π− p → π+π−n and π− p → π0π− p. The validity of the Vector Meson Dominance model will be clarified by the combined use of both hadronic and di-electron channels
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