Studying Time-Like Electromagnetic Baryon Transitions with HADES in $\pi \hbox {N}$ Reactions

Abstract

A dedicated program to study electromagnetic baryonic transitions in the time-like region has started using the pion beam and the High Acceptance Di-Electron Spectrometer set-up at GSI. Combined measurements of hadronic and dielectron final states have been performed for the first time 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 of the Bonn–Gatchina group together with the world data from pion and photon beam facilities. The obtained solution provides the excitation function of the two-pion production in photo- and pion- induced reactions around the pole of the D$_{13}$ resonance N(1520) and of its decomposition into the different resonant, non-resonant and $\rho$ contributions. Moreover, the exclusive $ne^+e^-$ production gives insight into time-like electromagnetic structure of baryons using both $e^+e^-$ invariant masses and angular distributions as most sensitive observables for model comparison. In addition, the extraction of the $\rho$ contribution in the two-pion production channels can be used to investigate the validity of the Vector Dominance Model for electromagnetic baryon transitions. The prospects for future experiments in the third resonance region is also discussed.