Inclusive e$^+$e$^-$ production has been studied with HADES in $\pi^-$ + p, $\pi^-$ + C and $\pi^- + \mathrm{CH}_2$ reactions, using the GSI pion beam at $\sqrt{s_{\pi p}}$ = 1.49 GeV. Invariant mass and transverse momentum distributions have been measured and reveal contributions from Dalitz decays of $\pi^0$, $\eta$ mesons and baryon resonances. The transverse momentum distributions are very sensitive to the underlying kinematics of the various processes. The baryon contribution exhibits a deviation up to a factor seven from the QED reference expected for the dielectron decay of a hypothetical point-like baryon with the production cross section constrained from the inverse $\gamma$ n$\rightarrow \pi^-$ p reaction. The enhancement is attributed to a strong four-momentum squared dependence of the time-like electromagnetic transition form factors as suggested by Vector Meson Dominance (VMD). Two versions of the VMD, that differ in the photon-baryon coupling, have been applied in simulations and compared to data. VMD1 (or two-component VMD) assumes a coupling via the $\rho$ meson and a direct coupling of the photon, while in VMD2 (or strict VMD) the coupling is only mediated via the $\rho$ meson. The VMD2 model, frequently used in transport calculations for dilepton decays, is found to overestimate the measured dielectron yields, while a good description of the data can be obtained with the VMD1 model assuming no phase difference between the two amplitudes. Similar descriptions have also been obtained using a time-like baryon transition form factor model where the pion cloud plays the major role.

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.

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 (pK^+Σ ^0) = 17.7 ± 1.7 (stat) ± 1.6 (syst) µb. Differential cross section distributions of the exclusive channel pp → pK^+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.

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.

The double differential production cross sections, $d^2σ/dΩdE$, for hydrogen isotopes and charged pions in the reaction of p + Nb at 3.5 GeV proton beam energy have been measured by the High Acceptance DiElectron Spectrometer (HADES). Thanks to the high acceptance of HADES at forward emission angles and usage of its magnetic field, the measured energy range of hydrogen isotopes could be significantly extended in comparison to the relatively scarce experimental data available in the literature. The data provide information about the development of the intranuclear cascade in the proton-nucleus collisions. They can as well be utilized to study the rate of energy/momentum dissipation in the nuclear systems and the mechanism of elementary and composite particle production in excited nuclear matter at normal density. Data of this type are important also for technological and medical applications. Our results are compared to models developed to describe the processes relevant to nuclear spallation (INCL++) or oriented to probe either the elementary hadronic processes in nuclear matter or the behavior of compressed nuclear matter (GiBUU).

We discuss new experimental results on the mechanisms of light nuclei production, fluctuations of conserved charges and the emissivity of matter, studied with HADES at SIS18. The multi-differential representations of hadron and dilepton spectra, collective effects and particle correlations are confronted with hitherto model calculations.

In nuclear collisions the incident protons generate a Coulomb field which acts on produced charged particles. The impact of these interactions on charged-pion transverse-mass and rapidity spectra, as well as on pion–pion momentum correlations is investigated in Au + Au collisions at $$\sqrt{s_\text {NN}}$$ = 2.4 GeV. We show that the low- $$m_t$$ region ( $$m_t < 0.2$$   GeV/ $$c^2$$ ) can be well described with a Coulomb-modified Boltzmann distribution that also takes changes of the Coulomb field during the expansion of the fireball into account. The observed centrality dependence of the fitted mean Coulomb potential energy deviates strongly from a $$A_{\text {part}}^{2/3}$$ scaling, indicating that, next to the fireball, the non-interacting charged spectators have to be taken into account. For the most central collisions, the Coulomb modifications of the HBT source radii are found to be consistent with the potential extracted from the single-pion transverse-mass distributions. This finding suggests that the region of homogeneity obtained from two-pion correlations coincides with the region in which the pions freeze-out. Using the inferred mean-square radius of the charge distribution at freeze-out, we have deduced a baryon density, in fair agreement with values obtained from statistical hadronization model fits to the particle yields.

The global polarization of Lambda hyperons along the total orbital angular momentum of a relativistic heavy-ion collision is presented based on the high statistics data samples collected in Au+Au collisions at root s(NN)= 2.4GeV and Ag+Ag at 2.55 GeV with the High-Acceptance Di-Electron Spectrometer (HADES) at GSI, Darmstadt. This is the first measurement below the strangeness production threshold in nucleon-nucleon collisions. Results are reported as a function of the collision centrality as well as a function of the hyperon's transverse momentum (p(T)) and rapidity (y(CM)) for the range of centrality 0-40%. We observe a strong centrality dependence of the polarization with an increasing signal towards peripheral collisions. For mid-central (20 - 40%) collisions the polarization magnitudes are < P-Lambda >(%) = 6.8 +/- 1.3 (stat.) +/- 2.1 (syst.) for Au+Au and < P-Lambda >(%) = 6.2 +/- 0.4 (stat.) +/- 0.6 (syst.) for Ag+Ag, which are the largest values observed so far. This observation thus provides a continuation of the increasing trend previously observed by STAR and contrasts expectations from recent theoretical calculations predicting a maximum in the region of collision energies about 3GeV. The observed polarization is of a similar magnitude as predicted by 3D-fluid-dynamics and the UrQMD plus thermal vorticity model and significantly above results from the AMPT model. (c) 2022 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

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

Adamczewski-Musch, J.; Arnold, O.; Atomssa, E. T.; Behnke, C.; Belounnas, A.; Belyaev, A.; Berger-Chen, J. C.; Biernat, J.; Blanco, A.; Blume, C.; Böhmer, M.; Chernenko, S.; Chlad, L.; Chudoba, P.; Ciepał, I.; Deveaux, C.; Dittert, D.; Dreyer, J.; Epple, E.; Fabbietti, L.; Fateev, O.; Fonte, P.; Franco, C.; Friese, J.; Fröhlich, I.; Galatyuk, T.; Garzón, J. A.; Gernhäuser, R.; Golubeva, M.; Greifenhagen, R.; Guber, F.; Gumberidze, M.; Harabasz, S.; Heinz, T.; Hennino, T.; Höhne, C.; Holzmann, R.; Ierusalimov, A.; Ivashkin, A.; Kämpfer, B.; Kardan, B.; Koenig, I.; Koenig, W.; Kolb, B. W.; Korcyl, G.; Kornakov, G.; Kornas, F.; Kotte, R.; Kuboś, J.; Kugler, A.; Kunz, T.; Kurepin, A.; Kurilkin, A.; Kurilkin, P.; Ladygin, V.; Lalik, R.; Lapidus, K.; Lebedev, A.; Linev, S.; Lopes, L.; Lorenz, M.; Mahmoud, T.; Maier, L.; Malige, A.; Markert, J.; Maurus, S.; Metag, V.; Michel, J.; Mihaylov, D. M.; Mikhaylov, V.; Morozov, S.; Müntz, C.; Münzer, R.; Naumann, L.; Nowakowski, K.; Parpottas, Y.; Pechenov, V.; Pechenova, O.; Petukhov, O.; Pietraszko, J.; Prozorov, A. P.; Przygoda, W.; Ramstein, B.; Rathod, N.; Reshetin, A.; Rodriguez-Ramos, P.; Rost, A.; Sadovsky, A.; Salabura, P.; Scheib, T.; SchmidtSommerfeld, K.; Schuldes, H.; Schwab, E.; Scozzi, F.; Seck, F.; Sellheim, P.; Siebenson, J.; Silva, L.; Sing, U.; Smyrski, J.; Spataro, S.; Spies, S.; Ströbele, H.; Stroth, J.; Strzempek, P.; Sturm, C.; Svoboda, O.; Szala, M.; Tlusty, P.; Traxler, M.; Tsertos, H.; Ungethüm, C.; Vazquez-Doce, O.; Wagner, V.; Wendisch, C.; Wiebusch, M. G.; Wirth, J.; Wójcik, D.; Zanevsky, Y.; Zumbruch, P.;

Currently, the HADES spectrometer undergoes un upgrade pro gram to be prepared for measurements at the upcoming SIS-100 synchrotron at FAIR. We de scribe the current status of the HADES di-electron measurements at the SIS-18 and our future plans for SIS-100. 1 Current experimental status The experimental determination of hadron properties insid e trongly interacting media (normal nuclear or hot and dense matter) is one of the very interestin g challenges in hadronic physics (see [1] for a review). As the strong coupling αs becomes very large below the QCD scale ΛQCD ≈ 200 MeV, non-perturbative effects as confinement and the breaki ng of chiral symmetry rule the nature of strong interactions in the universe (besi de extreme cases like neutron stars) solely. Experimentally, the approach to get a deeper unders tanding of these features is to create nuclear matter under extreme conditions in the laboratory w hich can be done by employing heavy ion reactions, only. During the evolution of the fireball, created in the course of heavy ion collisions, the properties of nuclear matter change drastically (see [2] for a deta iled discussion): starting as a hot and dense intermediate state eventually with “free” ( i.e., only partly or weakly bound) quarks and gluons the system cools down until the exchange of resonance species stops (chemical freezeout) and finally the particles do not scatter any longer elast ic lly (thermal freeze-out). Our goal is, however, to gather information of the early phase, i.e. before the chemical freeze-out occurs. One of the main probes (if not the only one) which carry undist orted information over the entire history are di-leptons ( ee orμμ) as they are not hampered by final-state interactions. This included the first-chance radiation (bremsstrahlung) , radiation from short-lived resonances (the real “messengers” which have a life time shorter then th ose of the fireball) and post-freezeout sources, such as the late π and η Dalitz decays, which can be subtracted if their yields and distributions at the freeze-out point are precisely kno wn. Now, after a successful decade of measurements of electromagnetic probes (summarized in [ 3, 4]) there is common agreement that only by systematic studies in various systems over a wid e energy range, conclusions of the Future perspectives at SIS-100 with HADES-at-FAIR 3 following questions can be drawn: when does the onset of deco nfinement appear, and how it is related to chiral symmetry restoration? Recently, HADES [5] has added valuable measurements of di-e lectron mass spectra for light A + A [6] systems as well as for (quasi-free) p + N collisions [7] at kinetic beam energies of 1-2 AGeV and 3.5 GeV, which has triggered a lot of theoretic al activities [8]. One of the main features of HADES is that at the same time it measures als o h drons: pions for absolute normalization [9] and hadrons containing strangeness [10] . To summarize the setup at this point, HADES is a magnetic spec trometer, consisting of up to 4 planes of Mini Drift Chambers (MDC) with a toroidal field c reated by a superconducting magnet. Particle identification is based on momentum and tim e-of-flight measurements. In addition, a Ring Imaging Cherenkov detector (RICH) and an elec tromagnetic Pre-Shower detector provide electron identification capabilities. HADES will continue its program at its current place at SIS-1 8, and then move to the upcoming FAIR accelerator complex. Here, HADES will continue its experimental program up to kinetic beam energies per nucleon of 8 GeV at SIS-100. This is one of the main reason for upgrading the HADES detector and its trigger and readout sys tem, which will be outlined in the following. 2 The upgrade program for HADES-at-FAIR

G. Agakishiev, A. Balanda, B. Bannier, R. Bassini , D. Belver, A.V. Belyaev, A. Blanco, M. Böhmer, J. L. Boyard, P. Braun-Munzinger , P. Cabanelas , E. Castro, S. Chernenko , T. Christ, M. Destefanis , J. Dı́az, F. Dohrmann, A. Dybczak, T. Eberl , W. Enghardt , L. Fabbietti , O.V. Fateev, P. Finocchiaro, P. Fonte, J. Friese, I. Fröhlich, T. Galatyuk, J. A. Garzón, R. Gernhäuser , A. Gil, C. Gilardi, M. Golubeva, D. González-Dı́az , F. Guber , M. Heilmann, T. Heinz, T. Hennino, R. Holzmann, A. Ierusalimov, I. Iori, A. Ivashkin, M. Jurkovic, B. Kämpfer, K. Kanaki, T. Karavicheva, D. Kirschner, I. Koenig, W. Koenig, B. W. Kolb, R. Kotte, F. Krizek, R. Krücken, W. Kühn, A. Kugler, A. Kurepin, S. Lang, J. S. Lange , K. Lapidus, T. Liu, L. Lopes, M. Lorenz, L. Maier, A. Mangiarotti , J. Markert , V. Metag, B. Michalska, J. Michel , D. Mishra, E. Morinière, J. Mousa, C. Müntz, L. Naumann, J. Otwinowski , Y. C. Pachmayer , M. Palka, Y. Parpottas , V. Pechenov , O. Pechenova , T. Pérez Cavalcanti , J. Pietraszko , W. Przygoda, B. Ramstein, A. Reshetin, M. Roy-Stephan , A. Rustamov, A. Sadovsky, B. Sailer, P. Salabura , A. Schmah, E. Schwab, Yu.G. Sobolev, S. Spataro , B. Spruck, H. Ströbele, J. Stroth, C. Sturm, M. Sudol , A. Tarantola, K. Teilab, P. Tlusty, M. Traxler, R. Trebacz , H. Tsertos, V. Wagner, M. Weber, M. Wisniowski, T. Wojcik, J. Wüstenfeld, S. Yurevich, Y.V. Zanevsky, P. Zhou, P. Zumbruch

A feasibility study has been performed in order to investigate the performance of the HADES  detector to measure the electromagnetic decays of the hyperon resonances $$\Sigma (1385)^{0}$$ ,   $$\Lambda (1405)$$   and $$\Lambda (1520)$$   as well as the production of double strange baryon systems $$\Xi ^{-}$$  and  $$\Lambda $$ $$\Lambda $$  in p + p reactions at a beam kinetic energy of $$4.5\,{\mathrm{GeV}}$$ . The existing HADES  detector will be upgraded by a new Forward Detector, which extends the detector acceptance into a range of polar angles that plays a crucial role for these investigations. The analysis of each channel is preceded by a consideration of the production cross-sections. Afterwards the expected signal count rates using a target consisting of either liquid hydrogen or polyethylene are summarized.

AbstractWe present high-statistic data on charged-pion emission from Au + Au collisions at $$\sqrt{s_{\mathrm{NN}}} = 2.4~\hbox {GeV}$$ s NN = 2.4 GeV (corresponding to $$E_{beam} = 1.23~\hbox {A GeV}$$ E beam = 1.23 A GeV ) in four centrality classes in the range 0–40% of the most central collisions. The data are analyzed as a function of transverse momentum, transverse mass, rapidity, and polar angle. Pion multiplicity per participating nucleon decreases moderately with increasing centrality. The polar angular distributions are found to be non-isotropic even for the most central event class. Our results on pion multiplicity fit well into the general trend of the available world data, but undershoot by $$2.5~\sigma $$ 2.5 σ data from the FOPI experiment measured at slightly lower beam energy. We compare our data to state-of-the-art transport model calculations (PHSD, IQMD, PHQMD, GiBUU and SMASH) and find substantial differences between the measurement and the results of these calculations.

We present high-statistic data on charged-pion emission from Au + Au collisions at √(s_NN) = 2.4 GeV (corresponding to E_beam = 1.23 A GeV ) in four centrality classes in the range 0–40

We present high-statistic data on charged pion emission from Au+Au collisions at $\sqrt{s_{\rm{NN}}}$ = 2.4 GeV (corresponding to $E_{beam}$ = 1.23 A GeV) in four centrality classes in the range 0 - 40$\%$ of the most central collisions. The data are analyzed as a function of transverse momentum, transverse mass, rapidity, and polar angle. Pion multiplicity per participating nucleon decreases moderately with increasing centrality. The polar angular distributions are found to be non-isotropic even for the most central event class. Our results on pion multiplicity fit well into the general trend of the world data, but undershoot by $2.5 \sigma$ data from the FOPI experiment measured at slightly lower beam energy. We compare our data to state-of-the-art transport model calculations (PHSD, IQMD, PHQMD, GiBUU and SMASH) and find substantial differences between the measurement and the results of these calculations.

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.

High-statistics $$\pi ^-\pi ^-$$ and $$\pi ^+\pi ^+$$ femtoscopy data are presented for Au + Au collisions at $$\sqrt{s_\mathrm{NN}} = 2.4~\hbox {GeV}$$, measured with HADES at SIS18/GSI. The experimental correlation functions allow the determination of the space-time extent of the corresponding emission sources via a comparison to models. The emission source, parametrized as three-dimensional Gaussian distribution, is studied in dependence on pair transverse momentum, azimuthal emission angle with respect to the reaction plane, collision centrality and beam energy. For all centralities and transverse momenta, a geometrical distribution of ellipsoidal shape is found in the plane perpendicular to the beam direction with the larger extension perpendicular to the reaction plane. For large transverse momenta, the corresponding eccentricity approaches the initial eccentricity. The eccentricity is smallest for most central collisions, where the shape is almost circular. The magnitude of the tilt angle of the emission ellipsoid in the reaction plane decreases with increasing centrality and increasing transverse momentum. All source radii increase with centrality, largely exhibiting a linear rise with the cube root of the number of participants. A substantial charge-sign difference of the source radii is found, appearing most pronounced at low transverse momentum. The extracted source parameters are consistent with the extrapolation of their energy dependence down from higher energies.

Pion-induced reactions provide unique opportunities for an unambiguous description of baryonic resonances and their coupling channels by means of a partial-wave analysis. Using the secondary pion beam at SIS18, the two-pion production in the second resonance region has been investigated to unravel the role of the N(1520) 3/2(-) resonance in the intermediate rho-meson production. Results on exclusive channels with one pion (pi(-)p) and two pions (pi(+)pi(-)n, pi(0)pi(-)p) in the final state measured in the pi(-)-p reaction at four different pion beam momenta (0.650, 0.685, 0.733, and 0.786 GeV/c) are presented. The excitation function of the different partial waves and Delta pi, N sigma, and N rho isobar configurations is obtained, using the Bonn-Gatchina partial-wave analysis. The N(1520)3/2(-) resonance is found to dominate the N rho final state with the branching ratio BR = 12.2% +/- 1.9%.

We present high-statistic data on charged-pion emission from Au + Au collisions at $$\sqrt{s_{\mathrm{NN}}} = 2.4~\hbox {GeV}$$ (corresponding to $$E_{beam} = 1.23~\hbox {A GeV}$$) in four centrality classes in the range 0–40% of the most central collisions. The data are analyzed as a function of transverse momentum, transverse mass, rapidity, and polar angle. Pion multiplicity per participating nucleon decreases moderately with increasing centrality. The polar angular distributions are found to be non-isotropic even for the most central event class. Our results on pion multiplicity fit well into the general trend of the available world data, but undershoot by $$2.5~\sigma $$ data from the FOPI experiment measured at slightly lower beam energy. We compare our data to state-of-the-art transport model calculations (PHSD, IQMD, PHQMD, GiBUU and SMASH) and find substantial differences between the measurement and the results of these calculations.