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The Dalitz decay of baryonic resonances (B\\[\\rightarrow \\] N\\[e^+e^-\\]) gives access to the time-like electromagnetic structure of baryonic transitions at small positive values of the squared four momentum transferred. Such processes were investigated by the HADES collaboration in elementary hadronic interactions. The Dalitz decay branching ratio of the \\[\\varDelta (1232)\\] resonance has been measured for the first time in pp reactions at 1.25 GeV beam energy. At higher energies, the \\[e^+e^-\\] emission from baryonic decays is shown to be sensitive to transition form factors of the Vector Dominance Model type. Using a pion beam, Dalitz decays of baryonic resonances can be measured more directly. In addition, the simultaneous measurement of hadronic channels provides a new data base for baryon spectroscopy issues. Preliminary results of a test experiment in the N(1520) region are shown, together with prospects for higher resonance studies. A project of measuring hyperon Dalitz decays at GSI and later on at FAIR is also discussed. The study of Dalitz decays of baryon resonances and, in particular, the role of the \\[\\rho \\] meson brings useful constraints for the study of medium effects also investigated by the HADES collaboration in reactions with nuclei.

We present the design of the simulation package Pluto, aimed at the study of hadronic interactions at SIS and FAIR energies. Its main mission is to offer a modular framework with an object-oriented structure, thereby making additions such as new particles, decays of resonances, new models up to modules for entire changes easily applicable. Overall consistency is ensured by a plugin- and distribution manager. Particular features are the support of a modular structure for physics process descriptions, and the possibility to access the particle stream for on-line modifications. Additional configuration and self-made classes can be attached by the user without re-compiling the package, which makes Pluto extremely configurable.

The production of Σ 0 hyperons in proton proton collisions at a beam kinetic energy of 3.5 GeV impinging on a liquid hydrogen target was investigated using data collected with the HADES setup. The total production cross section is found to be σ ( p K + Σ 0 ) = 17.7 ± 1.7 ( s t a t ) ± 1.6 ( s y s t ) µb. Differential cross section distributions of the exclusive channel p p → p K + Σ 0 were analyzed in the center-of-mass, Gottfried–Jackson and helicity reference frames for the first time at the excess energy of 556 MeV. The data support the interplay between pion and kaon exchange mechanisms and clearly demonstrate the contribution of interfering nucleon resonances decaying to K + Σ 0 . The Bonn–Gatchina partial wave analysis was employed to analyse the data. Due to the limited statistics, it was not possible to obtain an unambiguous determination of the relative contribution of intermediate nucleon resonances to the final state. However nucleon resonances with masses around 1.710 GeV / c 2 ( N ∗ ( 1710 ) ) and 1.900 GeV / c 2 ( N ∗ ( 1900 ) or Δ ∗ ( 1900 ) ) are preferred by the fit.

The production of$$\\Sigma ^{0}$$Σ 0 hyperons in proton proton collisions at a beam kinetic energy of 3.5 GeV impinging on a liquid hydrogen target was investigated using data collected with the HADES setup. The total production cross section is found to be$${\\sigma (pK^{+}\\Sigma ^{0}) = 17.7 \\pm 1.7 (stat) \\pm 1.6 (syst)}$$σ ( p K + Σ 0 ) = 17.7 ± 1.7 ( s t a t ) ± 1.6 ( s y s t ) µb. Differential cross section distributions of the exclusive channel$${pp \\rightarrow pK^{+}\\Sigma ^{0}}$$p p → p K + Σ 0 were analyzed in the center-of-mass, Gottfried–Jackson and helicity reference frames for the first time at the excess energy of 556 MeV. The data support the interplay between pion and kaon exchange mechanisms and clearly demonstrate the contribution of interfering nucleon resonances decaying to$$\\textrm{K}^{+}\\Sigma ^{0}$$K + Σ 0 . The Bonn–Gatchina partial wave analysis was employed to analyse the data. Due to the limited statistics, it was not possible to obtain an unambiguous determination of the relative contribution of intermediate nucleon resonances to the final state. However nucleon resonances with masses around 1.710$${\\textrm{GeV}/\\textrm{c}^{2}}$$GeV / c 2 ($${\\textrm{N}^{*}(1710)}$$N ∗ ( 1710 ) ) and 1.900$${\\textrm{GeV}/\\textrm{c}^{2}}$$GeV / c 2 ($${\\textrm{N}^{*}(1900)}$$N ∗ ( 1900 ) or$${\\Delta ^{*}(1900)}$$Δ ∗ ( 1900 ) ) are preferred by the fit.

High-precision measurements of flow coefficients$$v_{n}$$v n ($$n = 1 - 4$$n = 1 - 4 ) for protons, deuterons and tritons relative to the first-order spectator plane have been performed in Au+Au collisions at$$\\sqrt{s_{_{{\\text {NN}}}= 2.4$$s NN = 2.4  GeV with the High-Acceptance Di-Electron Spectrometer (HADES) at the SIS18/GSI. Flow coefficients are studied as a function of transverse momentum$$p_{{\\text {t}}}$$p t and rapidity$$y_{{\\text {cm}}}$$y cm over a large region of phase-space and for several classes of collision centrality. A clear mass hierarchy, as expected by relativistic hydrodynamics, is found for the slope of$$v_{1}$$v 1 ,$$d v_{1}/d y^{\\prime }|_{y^{\\prime } = 0}$$d v 1 / d y ′ | y ′ = 0 where$$y^{\\prime }$$y ′ is the scaled rapidity, and for$$v_{2}$$v 2 at mid-rapidity. Scaling with the number of nucleons is observed for the$$p_{{\\text {t}}}$$p t  dependence of$$v_{2}$$v 2 and$$v_{4}$$v 4 at mid-rapidity, which is indicative for nuclear coalescence as the main process responsible for light nuclei formation.$$v_{2}$$v 2 is found to scale with the initial eccentricity$$\\langle \\epsilon _{2} \\rangle $$⟨ ϵ 2 ⟩ , while$$v_{4}$$v 4 scales with$$\\langle \\epsilon _{2} \\rangle ^{2}$$⟨ ϵ 2 ⟩ 2 and$$\\langle \\epsilon _{4} \\rangle $$⟨ ϵ 4 ⟩ . The multi-differential high-precision data on$$v_{1}$$v 1 ,$$v_{2}$$v 2 ,$$v_{3}$$v 3 , and$$v_{4}$$v 4 provides important constraints on the equation-of-state of compressed baryonic matter.

High precision measurements of flow coefficientsv_(n)( n = 1 - 4 ) for protons, deuterons and tritons relative to the first-order spectator plane have been performed in Au+Au collisions at√s̅_̅(̅N̅N̅)̅ = 2.4GeV with the High-Acceptance Di-Electron Spectrometer (HADES) at the SIS18/GSI. Flow coefficients are studied as a function of transverse momentump_(t)and rapidityy_(cm)over a large region of phase space and for several classes of collision centrality. A clear mass hierarchy is found for the slope ofv₁ ,d v₁/d y^(′)|_(y^(′) = 0)wherey^(′)is the scaled rapidity, and forv₂at mid-rapidity. Scaling with the number of nucleons is observed for thep_(t)dependence ofv₂andv₄at mid-rapidity, which is indicative for nuclear coalescence as the main process responsible for light nuclei formation.v₂is found to scale with the initial eccentricity〈 ∊₂ 〉 , whilev₄scales with〈 ∊₂ 〉²and〈 ∊₄ 〉 . The multi-differential high-precision data onv₁ ,v₂ ,v₃ , andv₄provides important constraints on the equation-of-state of compressed baryonic matter.

The HADES collaboration uses the e + e − production as a probe of the resonance matter produced in collisions at incident energies of 1-3.5 GeV/nucleon at GSI. Elementary reactions provide useful references for these studies and give information on resonance Dalitz decays (R→Ne + e − ). Such processes are sensitive to the structure of time-like electromagnetic baryon transitions in a kinematic range where (off-shell) vector mesons play a crucial role. Results obtained in proton-proton reactions and in a commissioning pion-beam experiment are reported and prospects for future pion beam experiments and for first hyperon Dalitz decay measurements are described. The connection with the investigations of medium effects to be continued with HADES in the next years at SIS18 and SIS100 is also discussed.

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 q 2 in timelike region. A prominent enhancement in the respective electromagnetic transition Form Factors (etFF) at q 2 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.

This contribution aims to give a basic overview of the latest results regarding the production of resonances in different collision systems. The results were extracted from experimental data collected with HADES that is a multipurpose detector located at the GSI Helmholtzzentrum, Darmstadt. The main points discussed here are: the properties of the strange resonances Λ(1405) and Σ(1385), the role of Δ’s as a source of pions in the final state, the production dynamics reflected in form of differential cross sections, and the role of the ϕ meson as a source for K− particles.