Studying excited states of the nucleon with the HADES detector at GSI

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

Etude d'états excités du nucléon avec le détecteur HADES à GSI

The subject of this thesis is the analysisof dielectron (e⁺e⁻) production channels produced in pion induced reactions with the High Acceptance Di-Electron Spectrometer (HADES) at a center of mass energy close to 1.5 GeV, where baryon resonances in the second resonance region (N(1440), N(1520),...) can be excited. The objective is to bring information on electromagnetic baryon transitions in an unexplored kinematic region, where vector mesons (ρ, w) are expected to play an important role. These measurements can also be used as a reference for studies of in-medium e⁺e⁻ production, investigating the modifications of the ρ meson spectral function. The document begins with a presentation of the results already obtained by the HADES collaboration concerning dielectron production. 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 is then discussed, together with the problem of subtracting the carbon contribution from the polyethylene target to study pion-nucleon interactions. The existing tools for the interpretation of the data are then presented.In addition to existing models for the π⁻ρ →ne⁺e⁻ reaction, the results of Partial Wave analysis of then γn→ π⁻ρ reaction as well as for π⁻ρ →2 πN 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.Le sujet de cette thèse est l’analyse de la production de di-électrons dans des réactions induites par des pions avec le détecteur HADES a une énergie dans le centre de masse proche de 1.5 GeV, dans lesquelles des résonances baryoniques dans la deuxième région de résonances (N(1440), N(1520),..) peuvent être excitées. L’objectif est d’obtenir des informations sur les transitions électromagnétiques dans une région inexplorée cinématiquement, où l’on s’attend à ce que les mésons vecteurs jouent un rôle important. Ces mesures peuvent aussi être utilisées comme référence pour les études de production de paires e⁺e⁻ dans la matière hadronique visant à explorer les modifications de la fonction spectrale du ρ. Le document commence par une présentation des résultats déjà obtenus par la collaboration HADES pour la production de di- électrons. Suit une description de l’expérience HADES et de la ligne de faisceau de pions. Les différentes étapes de l’analyse sont ensuite présentées, en insistant sur l’identification des électrons. Une étude détaillée de l’ajustement de la réponse du détecteur RICH, qui est crucial pour la discrimination entre électrons et pions est réalisée. Puis, l’extraction du signal de production de di-électrons, ainsi que les problèmes de soustraction de la contribution de noyaux de carbone pour étudier les interactions pion-nucléon. Les outils existants pour l’interprétation des données sont ensuite introduits. En plus des modèles existants pour la réaction π⁻ρ →ne⁺e⁻, les résultats de l’analyse en ondes partielles des canaux 2πN peuvent être utilisés. La distribution de la masse invariante des paires e⁺e⁻ présente un excès par rapport à la production attendue pour des baryons ponctuels, ce qui confirme le rôle important de mésons _ hors-couche dans les facteurs de forme de transition électromagnétique dans ce domaine cinématique. Les résultats de l’analyse des distributions angulaires en utilisant le formalisme de la matrice densité sont aussi comparés`a un modèle théorique basé sur les contributions de plusieurs résonances. Cette comparaison montre que les distributions angulaires apportent des informations supplémentaires sur la structure des transitions baryoniques électromagnétiques. Cette analyse est utilisée pour motiver des expériences futures pour étudier les transitions électromagnétiques mettant enjeu des baryons non-étranges

Resonance production and decay in pion induced collisions with HADES

The main goal of the High Acceptance Di-Electron experiment (HADES) at GSI is the study of hadronic matter in the 1-3.5 GeV/nucleon incident energy range. HADES results on e+ e− production in proton-nucleus reactions and in nucleus-nucleus collisions demonstrate a strong enhancement of the dilepton yield relative to a reference spectrum obtained from elementary nucleon-nucleon reactions. These observations point to a strong modification of the in-medium ρ spectral function driven by the coupling of the ρ to baryon-resonance hole states. However, to scrutinize this conjecture, a precise study of the role of the ρ meson in electromagnetic baryon-resonance transitions in the time-like region is mandatory. In order to perform this study, HADES has started a dedicated pion-nucleon programme. For the first time a combined measurement of hadronic and dielectron final states have been performed in π−-N reactions at four different pion beam momenta (0.656, 0.69, 0.748 and 0.8 GeV/c). First results on exclusive channels with one pion π−-p and two pions (nπ+π−, pπ−π0) in the final state, which are currently analysed within the partial wave analysis (PWA) framework of the Bonn-Gatchina group, are discussed. Finally first results for the dielectron analysis will be shown

The antiproton interaction with an internal 12C target inside the HESR ring at FAIR

In order to fulfill the goal of producing higher rates of doubly strange hyperons, the ¯PANDA collaboration will use the antiproton ring HESR at the future facility FAIR. The low energy hyperon production by an antiproton beam requires to insert a solid target inside the ring. Unwanted side effects of such an insertion are the overwhelming amount of annihilations, which would make the detectors blind, and the fast depletion of the bunch, which circulates inside the ring. The choice of the target material impacts the hyperon production yield: Carbon turned out to provide enough initial hyperon deceleration and keep secondary interactions below a tolerable level. The use of a very thin Diamond target, together with beam steering techniques, seems to be a satisfactory solution to the above problems and will be described hereafter

Exploring time like tranistions in pp, πp and AA reactions with HADES

Radiative transition of an excited baryon to a nucleon with emission of a virtual massive photon converting to dielectron pair (Dalitz decays) provides important information about baryon-photon coupling at low q2 in timelike region. A prominent enhancement in the respective electromagnetic transition Form Factors (etFF) at q2 near vector mesons ρ/ω poles has been predicted by various calculations reflecting strong baryon-vector meson couplings. The understanding of these couplings is also of primary importance for the interpretation of the emissivity of QCD matter studied in heavy ion collisions via dilepton emission. Dedicated measurements of baryon Dalitz decays in proton-proton and pion-proton scattering with HADES detector at GSI/FAIR are presented and discussed. The relevance of these studies for the interpretation of results obtained from heavy ion reactions is elucidated on the example of the HADES results

Correlated pion-proton pair emission off hot and dense QCD matter

In this letter we report the first multi-differential measurement of correlated pion-proton pairs from 2 billion Au+Au collisions at sNN=2.42 GeV collected with HADES. In this energy regime the population of Δ(1232) resonances plays an important role in the way energy is distributed between intrinsic excitation energy and kinetic energy of the hadrons in the fireball. The triple differential d3N/dMπ±pdpTdy distributions of correlated π±p pairs have been determined by subtracting the πp combinatorial background using an iterative method. The invariant-mass distributions in the Δ(1232) mass region show strong deviations from a Breit-Wigner function with vacuum width and mass. The yield of correlated pion-proton pairs exhibits a complex isospin, rapidity and transverse-momentum dependence. In the invariant mass range 1.1<Minv(GeV/c2)<1.4, the yield is found to be similar for π+p and π−p pairs, and to follow a power law 〈Apart〉α, where 〈Apart〉 is the mean number of participating nucleons. The exponent α depends strongly on the pair transverse momentum (pT) while its pT-integrated and charge-averaged value is α=1.5±0.08st±0.2sy