title: Studying excited states of the nucleon with the HADES detector at GSI
Creators: name: Scozzi, Federico
Publisher: name: UNSPECIFIED
Year: 2020

description: The subject of this thesis is the analysis of dielectron (e+e-) production channels produced in pion induced reactions at a center of mass energy close to 1.5 GeV, where baryon resonances in the second resonance region (N(1440), N(1520), N(1535),...) can be excited. The objective is to bring information on electromagnetic baryon transitions in an unexplored kinematic region, where vector mesons (ρ, ω ) are expected to play an important role. These measurements can also be used as a reference for studies of in-medium e+e- production, aiming at investigating the modifications of the ρ meson spectral function at finite baryonic densities. While experimentally more challenging, pion-induced reactions constitute a more direct tool to study baryon transitions. The measurement of hadronic channels by the HADES collaboration in the pion-nucleon reactions allowed to enrich the very scarce existing data base and, using also the very rich photon-induced data, to perform a new Partial Wave Analysis to extract information on baryonic resonance properties. The results of this analysis are used as a valuable input for the analysis of the electromagnetic channels. The present analysis is based on a commissioning experiment carried out with the High Acceptance Di-Electron Spectrometer (HADES), using the GSI secondary pion beam impinging on carbon and polyethylene targets. The document begins with a presentation of the results already obtained by the HADES collaboration concerning dielectron production in nucleus-nucleus, nucleon-nucleus or nucleon-nucleon and of the scarce existing results for pion-induced reactions. It is followed by a description of the HADES experiment and of the pion beam line. Then the different steps of the data analysis are presented, with a focus on the electron identification. For this purpose a careful study on how to tune in simulation the response of the RICH detector, fundamental for the discrimination between pions and electrons, is performed. The extraction of the signal for dielectron production, including combinatorial background subtraction and normalization is then discussed, together with the problems of subtracting the carbon contribution from the polyethylene target to study pion-nucleon interactions. All the contributions to the efficiency of the measurement are carefully taken into account and the systematic errors are estimated. The existing tools for the interpretation of the data are then presented. In addition to existing models for the π−p → ne+ e− reaction, the results of Partial Wave analysis of the γn →π −p reaction as well as for π− p → 2πN channels can be used. The e+e− invariant mass distribution shows a clear enhancement with respect to a production by point-like baryons, and confirms the significant role of off-shell ρ mesons in electromagnetic form factors in this kinematic range. Results of the analysis of the angular distributions using the framework of the spin density matrix are also compared to a theoretical model based on several resonance contributions. This demonstrates the additional information provided by angular distributions to extract information on electromagnetic structure of baryon transitions. This analysis is used to motivate further experiments to study electomagnetic transitions involving non-strange baryons in the third resonanceregion or hyperons.