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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.

About 10 μs after the Big Bang, the universe was filled—in addition to photons and leptons—with strong-interaction matter consisting of quarks and gluons, which transitioned to hadrons at temperatures close to kT = 150 MeV and densities several times higher than those found in nuclei. This quantum chromodynamics (QCD) matter can be created in the laboratory as a transient state by colliding heavy ions at relativistic energies. The different phases in which QCD matter may exist depend for example on temperature, pressure or baryochemical potential, and can be probed by studying the emission of electromagnetic radiation. Electron–positron pairs emerge from the decay of virtual photons, which immediately decouple from the strong interaction, and thus provide information about the properties of QCD matter at various stages. Here, we report the observation of virtual photon emission from baryon-rich QCD matter. The spectral distribution of the electron–positron pairs is nearly exponential, providing evidence for a source of temperature in excess of 70 MeV with constituents whose properties have been modified, thus reflecting peculiarities of strong-interaction QCD matter. Its bulk properties are similar to the dense matter formed in the final state of a neutron star merger, as apparent from recent multimessenger observation.

High-precision measurements of flow coefficients v n ( n = 1 - 4 ) for protons, deuterons and tritons relative to the first-order spectator plane have been performed in Au+Au collisions at 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 t and rapidity 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 , d v 1 / d y ′ | y ′ = 0 where y ′ is the scaled rapidity, and for v 2 at mid-rapidity. Scaling with the number of nucleons is observed for the p t  dependence of v 2 and v 4 at mid-rapidity, which is indicative for nuclear coalescence as the main process responsible for light nuclei formation. v 2 is found to scale with the initial eccentricity ⟨ ϵ 2 ⟩ , while v 4 scales with ⟨ ϵ 2 ⟩ 2 and ⟨ ϵ 4 ⟩ . The multi-differential high-precision data on v 1 , v 2 , v 3 , and v 4 provides important constraints on the equation-of-state of compressed baryonic matter.

Hadron production ( π ± , proton, Λ , K S 0 , K ± ) in π - + C and π - + W collisions is investigated at an incident pion beam momentum of 1.7 GeV / c . This comprehensive set of data measured with HADES at SIS18/GSI significantly extends the existing world data on hadron production in pion induced reactions and provides a new reference for models that are commonly used for the interpretation of heavy-ion collisions. The measured inclusive differential production cross-sections are compared with state-of-the-art transport model (GiBUU, SMASH) calculations. The (semi-) exclusive channel π - + A → Λ + K S 0 + X , in which the kinematics of the strange hadrons are correlated, is also investigated and compared to a model calculation. Agreement and remaining tensions between data and the current version of the considered transport models are discussed.

In this paper we report on the investigation of baryonic resonance production in proton-proton collisions at the kinetic energies of 1.25 GeV and 3.5 GeV, based on data measured with HADES. Exclusive channels npπ+ and ppπ0 as well as ppe+e− were studied simultaneously in the framework of a one-boson exchange model. The resonance cross sections were determined from the one-pion channels for Δ(1232) and N(1440) (1.25 GeV) as well as further Δ and N* resonances up to 2 GeV/c2 for the 3.5 GeV data. The data at 1.25 GeV energy were also analysed within the framework of the partial wave analysis together with the set of several other measurements at lower energies. The obtained solutions provided the evolution of resonance production with the beam energy, showing a sizeable non-resonant contribution but with still dominating contribution of Δ(1232)P33. In the case of 3.5 GeV data, the study of the ppe+e− channel gave the insight on the Dalitz decays of the baryon resonances and, in particular, on the electromagnetic transition form-factors in the time-like region. We show that the assumption of a constant electromagnetic transition form-factors leads to underestimation of the yield in the dielectron invariant mass spectrum below the vector mesons pole. On the other hand, a comparison with various transport models shows the important role of intermediate ρ production, though with a large model dependency. The exclusive channels analysis done by the HADES collaboration provides new stringent restrictions on the parameterizations used in the models.

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.

. The combination of a production target for secondary beams, an optimized ion optical beam line setting, in-beam detectors for minimum ionizing particles with high rate capability, and an efficient large acceptance spectrometer around the reaction target constitutes an experimental opportunity to study in detail hadronic interactions utilizing pion beams impinging on nucleons and nuclei. For the 0.4-2.0GeV/c pion momentum regime such a facility is located at the heavy ion synchrotron accelerator SIS18 in Darmstadt (Germany). The layout of the apparatus, performance of its components and encouraging results from a first commissioning run are presented.

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.

We present and discuss recent experimental activities of the HADES collaboration on open and hidden strangeness production close or below the elementary NN threshold. Special emphasis is put on the feed-down from φ mesons to antikaons, the presence of the Ξ- excess in cold nuclear matter and the comparison of statistical model rates to elementary p+p data. The implications for the interpretation of heavy-ion data are discussed as well.

Hadron modifications in nuclear matter are discussed in connection to chiral symmetry restoration and/or hadronic many body effects. Experiments with photon, proton and heavy ion beams are used to probe properties of hadrons embedded in nuclear matter at different temperatures and densities. Most of the information has been gathered for the light vector mesons ρ ω and ø. HADES is a second generation experiment operating at GSI with the main aim to study in-medium modifications by means of dielectron production at the SIS18/Bevelac energy range. Large acceptance and excellent particle identification capabilities allows also for measurements of strangeness production. These abilities combined with the variety of beams provided by the SIS18 allow for a characterization of properties of the dense baryonic matter properties created in heavy ion collisions at these energies. A review of recent experimental results obtained by HADES is presented, with main emphasis on hadron properties in nuclear matter.