High Acceptance Di-Electron Spectrometer Research Articles

Production and decays of hyperons in p+p reactions measured with HADES

Abstract. Preliminary results of inclusive σ+(1385) production in p + p reactions at Ebeam = 4.5 GeV and Λ(1520) in p + p and p + Nb at Ebeam = 3.5 GeV are presented. Both measurements were performed by the HADES ("High Acceptance Di-Electron Spectrometer") experiment at FAIR/GSI. The analysis consisted of multiple steps including track reconstruction, particle identification and the reconstruction of the hyperon signal from their hadronic decays Λ(1520) → Λπ+π− and σ+(1385) → Λπ+. The main part of the analysis, which is signal/background classification, was performed utilizing Machine Learning methods. The employed method relied on a data-driven approach called “Classification Without Labels" and the Multilayer Perceptron style Artificial Neural Networks. The analysis resulted in reconstruction of the resonance distributions for both σ+(1385) and Λ(1520), and allowed for the extraction of parameters of the relativistic Breit-Wigner distributions.

Hyperon production and interaction studies in protonproton scattering with HADES

Hyperons and their interactions with baryonic matter may play a key role in our understanding of the core of neutron stars. Moreover, the internal structure of hyperons and their excitations are of great interest. Their structure can be probed by exploiting their couplings to radiative probes such as by measuring electromagnetic transition form factors. HADES (High-Acceptance Di-Electron Spectrometer) at GSI collected high-statistics proton-proton scattering data in 2022 at √s = 3.48 GeV. The Λ - Λ interaction is currently being studied in this data along with production of Σ states where the Dalitz decay of hyperons might be observed for the first time. In addition, the reaction pp → pK+K+Ξ−[π−Λ[pπ−]] has been studied to probe the production mechanism of double strangeness near its production threshold. This proceeding addresses the hyperon physics case and discusses the ongoing Ξ− analysis. A preliminary upper limit for the cross section at √s = 3.48 GeV is presented taking into account only statistical uncertainties.

Partial Wave Amplitude Analysis in Pion-Induced Reactions at the HADES Experiment

The High Acceptance Di-Electron Spectrometer (HADES) collaboration at GSI employs a pion beam to examine the characteristics of baryonic resonances and their decay channels. This pion-beam facility enables the generation of baryonic resonances at a fixed center of mass energy (√s), i.e. in the S-channel. In anticipation of conducting a more comprehensive exploration of the resonance regions in pion-proton collisions, a new implementation of the K-matrix & D-matrix frameworks is currently under development. This updated implementation aims to offer a refined mapping of these regions. Example fits are presented showing the current status and the potential of the new framework.

Performance of the new hadron blind HADES RICH in heavy ion collisions

The High Acceptance Dielectron Spectrometer (HADES) situated at GSI Darmstadt, Germany, has recently upgraded its gaseous ring imaging Cherenkov (RICH) detector. Studying the properties of the hot and dense fireballs produced in heavy ion collisions the RICH is the key component to efficiently identify electrons and positrons emitted therein. The new photon detection plane is comprised of 428 multianode photo electron multipliers (MAPMT)(Hamamatsu H12700) partly coated with p-Terphenyl as a wavelength shifter. It is operated with a gaseous C4H10 (isobutane) radiator making it essentially hadron blind for particle momenta up to approximately 2.5GeV/c. All 27392 MAPMT channels are read out by an FPGA based readout electronic scheme called DIRICH, which will also be used in other future Cherenkov detectors. The DIRICH readout allows to measure leading and trailing edges for each pixel pulse and hence time over threshold and hit arrival time down to sub-nanosecond precision. This article will discuss the key features of the upgraded HADES RICH and its performance based on a Ag+Ag run at E=1.58AGeV incident energy within the FAIR Phase-0 research program.

Proton, deuteron and triton flow measurements in Au+Au collisions at sqrt{s_{_{{text {NN}}}}}= 2.4 GeV

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 sqrt{s_{_{{text {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}}} and rapidity y_{{text {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^{prime }|_{y^{prime } = 0} where y^{prime } is the scaled rapidity, and for v_{2} at mid-rapidity. Scaling with the number of nucleons is observed for the p_{{text {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 langle epsilon _{2} rangle , while v_{4} scales with langle epsilon _{2} rangle ^{2} and langle epsilon _{4} rangle . 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 new HADES ToF Forward Detector

The High-Acceptance DiElectron Spectrometer (HADES) at GSI Darmstadt consists of a 6-coil toroidal magnet centered on the beam axis and six identical detection sections located between the coils and covering polar angles between 18° and 85°. The physics aims include the study of the properties of hot and dense hadronic matter as well as elementary and pion-induced reactions.To increase the acceptance of HADES at very low polar angles in the forward region, between 0.5∘ and 7°, a new detector, the Forward Detector (FD), has been built. The FD is composed of a tracking and a Time Of Flight (TOF) detector based on Resistive Plate Chamber (RPC) technology. The TOF detector, covering an area of around 2 m2, is composed by 128 strip-like shielded RPC cells, with two different widths 22mm and 44mm and 750mm length distributed in four modules symmetrically placed around the beam axis. Each cell is composed by four gas gaps, 0.270mm, delimited by three (2mm) aluminum and two (1mm) glass electrodes. In order to cope with an expected maximum particle load of around 400 Hz/cm2, close to the beam axis, the detector is operated above room temperature in order to decrease the resistivity of the glass and increase the count rate capability.Details of the system construction and results concerning timing precision are described in this communication. The detector was operated at 31.5 °C with a maximum particle load of around 600 Hz/cm2 during a production beam time for six weeks in early 2022 showing an average time precision of around 80 ps.

Measurement of global polarization of Λ hyperons in few-GeV heavy-ion collisions

The global polarization of Λ 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 sNN=2.4 GeV 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 (pT) and rapidity (yCM) 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 〈PΛ〉(%)=6.8±1.3(stat.)±2.1(syst.) for Au+Au and 〈PΛ〉(%)=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 3 GeV. 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.

LGAD technology for HADES, accelerator and medical applications

Low Gain Avalanche Diode (LGAD) technology has been used to design and construct prototype and full-size beam detector systems for applications requiring simultaneous time and spatial precision. For these purposes, a dedicated LGAD strip sensor production has been conducted at Fondazione Bruno Kessler (FBK) with different strip geometries and sizes. This contribution will review a wide variety of LGAD applications ranging from the reaction time (T0) detector for experiments utilizing proton and pion beams with the High Acceptance Di-Electron Spectrometer (HADES) at GSI in Darmstadt, Germany, to beam structure monitoring at the Superconducting DArmstadt LINear ACcelerator (S-DALINAC) at the Technische Universität Darmstadt operated in energy recovery mode and medical applications at the MedAustron facility in Wiener Neustadt, Austria. We will also give a prospect of further upgrade projects at GSI and FAIR facilities.

Directed, Elliptic, and Higher Order Flow Harmonics of Protons, Deuterons, and Tritons in Au+Au Collisions at sqrt[s_{NN}]=2.4 GeV.

Flow coefficients v_{n} of the orders n=1-6 are measured with the High-Acceptance DiElectron Spectrometer (HADES) at GSI for protons, deuterons, and tritons as a function of centrality, transverse momentum, and rapidity in Au+Au collisions at sqrt[s_{NN}]=2.4 GeV. Combining the information from the flow coefficients of all orders allows us to construct for the first time, at collision energies of a few GeV, a multidifferential picture of the angular emission pattern of these particles. It reflects the complicated interplay between the effect of the central fireball pressure on the emission of particles and their subsequent interaction with spectator matter. The high precision information on higher order flow coefficients is a major step forward in constraining the equation of state of dense baryonic matter.

Reconstruction of γγ mass spectra in Ag+Ag collisions at 1.23 and 1.58 AGeV beam energies with ECal detector of the HADES experiment

HADES is a high acceptance di-electron spectrometer operating at SIS18, GSI, Germany aimed at study of hadron-proton, hadron-nucleus and nucleus-nucleus collisions at 1-4 AGeV beam energies. The new electromagnetic calorimeter (ECal) was added to the experimental setup in order to measure γ-quanta and thus extend its capabilities in study of π 0-, η-mesons, production of neutral hyperons and to improve electron-to-hadron separation for the partcles with momenta p > 300 MeV/c. The first data taking with the ECal detector was carried out in March 2019 when Ag+Ag collisions at 1.23 AGeV and 1.58 AGeV beam energies were studied. The methods of reconstruction of the γγ invariant mass spectra from these data are discussed. The analysis includes several steps: calibration of each module of the ECal detector, identification of γ-quanta, reconstruction of γγ invariant mass spectra and subtraction of combinatorial background. The obtained results show experimental capabilities of the new detector and, after efficiency corrections, will allow to normalize yields of other particles.

Commissioning of the electromagnetic calorimeter ECAL of the HADES experiment

HADES (High Acceptance Di-Electron Spectrometer) is located at the GSI (Helmholtzzentrum für Schwerionenforschung) Darmstadt. It is an experiment focused on the study of the hot and dense nuclear matter mainly via the detection of the di-lepton pairs. Electromagnetic CALorimeter (ECAL) was recently added to the HADES setup. This new subdetector allows measuring of photons from the decay of neutral mesons and resonances. It also allows to discriminate between electrons and pions in the high-momenta region over 400 MeV/c. ECAL follows same hexagonal geometry as HADES, i.e. it consists of six sectors in azimuth. The first four sectors were finished and commissioned in 2018. The first experiment with ECAL included in HADES setup took place on March 2019, investigating the Ag+Ag reaction at beam energy of 1.65 A GeV. During the commissioning, several issues popped up and they were addressed. The issues and their solution will be described in the article.

Proton-number fluctuations in sNN=2.4 GeV Au + Au collisions studied with the High-Acceptance DiElectron Spectrometer (HADES)

We present an analysis of proton number fluctuations in $\sqrt{s_{NN}}$ = 2.4 GeV Au+Au collisions measured with the High-Acceptance DiElectron Spectrometer (HADES) at GSI. With the help of extensive detector simulations done with IQMD transport model events including nuclear clusters, various nuisance effects influencing the observed proton cumulants have been investigated. Acceptance and efficiency corrections have been applied as a function of fine grained rapidity and transverse momentum bins, as well as considering local track density dependencies. Next, the effects of volume changes within particular centrality selections have been considered and beyond-leading-order corrections have been applied to the data. The efficiency and volume corrected proton number moments and cumulants Kn of orders n = 1, . . . , 4 have been obtained as a function of centrality and phase-space bin, as well as the corresponding correlators C_n . We find that the observed correlators show a power-law scaling with the mean number of protons, i.e. $C_n \propto <N>^n$, indicative of mostly long-range multi-particle correlations in momentum space. We also present a comparison of our results with Au+Au collision data obtained at RHIC at similar centralities, but higher $\sqrt{s_{NN}}$.

Two-pion production in the second resonance region in π−p collisions with the High-Acceptance Di-Electron Spectrometer (HADES)

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)32− resonance in the intermediate ρ-meson production. Results on exclusive channels with one pion (π−p) and two pions (π+π−n, π0π−p) in the final state measured in the π−−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 Δπ, Nσ, and Nρ isobar configurations is obtained, using the Bonn-Gatchina partial-wave analysis. The N(1520)32− resonance is found to dominate the Nρ final state with the branching ratio BR=12.2%±1.9%.5 MoreReceived 20 April 2020Accepted 22 June 2020DOI:https://doi.org/10.1103/PhysRevC.102.024001©2020 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasMeson & hyperon induced nuclear reactionsNuclear many-body theoryParticle interactionsPhysical SystemsBaryonsNuclear Physics

Low Gain Avalanche Detectors for the HADES reaction time (T_0) detector upgrade

Low Gain Avalanche Detector (LGAD) technology has been used to design and construct prototypes of time-zero detector for experiments utilizing proton and pion beams with High Acceptance Di-Electron Spectrometer (HADES) at GSI Darmstadt, Germany. LGAD properties have been studied with proton beams at the COoler SYnchrotron facility in Jülich, Germany. We have demonstrated that systems based on a prototype LGAD operated at room temperature and equipped with leading-edge discriminators reach a time precision below 50 ps. The application in the HADES, experimental conditions, as well as the test results obtained with proton beams are presented.

Study of baryonic resonances andρmeson production in the reactionpp → ppπ+π-at 3.5 GeV with HADES

The High Acceptance Di-Electron Spectrometer (HADES) installed at GSI in Darmstadt and de signed to investigate dielectron production in heavy-ion collisions in the range of kinetic beam energies 1-3 A GeV is also an excellent detector for charged hadron detection. We discuss the analysis of thepp → ppπ+π-channel at an incident energy of 3.5 GeV, using results from pp→ppπ0, pp→pnπ+andpp → pKΛ measured at the same energy by HADES. A consistent description of the differential cross sections as a function of the invariant mass and the production angles is achieved by a simple resonance model taking into account the single and the double excitation of baryonic resonances with masses up to 1.9 GeV. The contributions can be quantified and confronted to the inputs of transport models or to the predictions of dedicated models for nucleon-nucleon reactions. In addition, using specific kinematical cuts, the total production cross section and the angular dis tribution of the ρ meson have been also extracted. As both baryon resonances and the ρ meson are important sources of di-electrons, these results will provide valuable constraints for the interpretation of the dielectron spectra measured by the HADES collaboration.

Studying time-like electromagnetic baryonic transitions with HADES in pion induced reactions

A dedicated programme aiming at studying electromagnetic baryonic transitions in the time-like region has started at GSI using the pion beam and the High Acceptance Di-Electron Spectrometer (HADES) set-up. A pioneering experiment has been carried out in the second resonance region, at a center-of-mass energy of 1.49 GeV, using carbon and polyethylene targets, allowing for an analysis of the inclusivee+e-production and of the exclusive quasi-freeπ-p→ne+e-reaction, in complement to hadronic channels. Predictions for the Dalitz decay of N(1520) and N(1535) based on real photon couplings strongly underestimate thee+e-yield at invariant masses larger than 300 MeV/c2, which signals effects of time-like baryon transition form factors, in qualitative agreement with the Vector Dominance Models (VDM). A quantitative description of the observede+e-yield is achieved by taking into account the contribution from off-shellρs. The latter was provided by the preliminary results of a Bonn-Gatchina Partial Wave Analysis of the two-pion production channels, which were measured simultaneously in our experiment. A good agreement is also obtained using a covariant model for the time-like electromagnetic form factors for the N-N(1520) transition. The angular distributions for thee+e-production contain additional selective information on the spin structure of the different transitions. The measurements with the pion beam will be extended in future in the third resonance region. Hyperon Dalitz decay studies in proton induced reactions are also foreseen.

Transverse momentum and rapidity dependence of collective flow harmonics of protons and deuterons in Au + Au reactions at 1.23 AGeV

AbstractWe explore the rapidity and transverse momentum dependence of collective flow harmonics of deuterons and protons in Au + Au reactions at a beam energy of 1.23 AGeV within the UrQMD approach. These investigations are of direct relevance for the high acceptance di electron spectrometer (HADES) experiment at the GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt, Germany, that has recently presented first data on the flow of light clusters in Au + Au collisions at 1.23 AGeV. To address the deuteron flow, UrQMD has been extended by a phase‐space coalescence approach. We find that this ansatz provides a very good description of the measured deuteron and proton flow data, if a hard equation of state is used for the simulation. The results show a strong impact of cluster formation on the observable collective flow and agree well with the data obtained by the HADES collaboration.

Studying Time-Like Electromagnetic Baryon Transitions with HADES in $$\pi \hbox {N}$$ π N Reactions

A dedicated program to study electromagnetic baryonic transitions in the time-like region has started using the pion beam and the High Acceptance Di-Electron Spectrometer set-up at GSI. Combined measurements of hadronic and dielectron final states have been performed for the first time in \(\pi \)-N reactions, using polyethylene and carbon targets. Differential cross sections of the exclusive channels with two pions in the final state (\(\pi ^-\pi ^+ n\), \(\pi ^0\pi ^- p\)) were obtained in the second resonance region with an unprecedented statistics. These new data were included in the partial wave analysis of the Bonn–Gatchina group together with the world data from pion and photon beam facilities. The obtained solution provides the excitation function of the two-pion production in photo- and pion- induced reactions around the pole of the D\(_{13}\) resonance N(1520) and of its decomposition into the different resonant, non-resonant and \(\rho \) contributions. Moreover, the exclusive \(ne^+e^-\) production gives insight into time-like electromagnetic structure of baryons using both \(e^+e^-\) invariant masses and angular distributions as most sensitive observables for model comparison. In addition, the extraction of the \(\rho \) contribution in the two-pion production channels can be used to investigate the validity of the Vector Dominance Model for electromagnetic baryon transitions. The prospects for future experiments in the third resonance region is also discussed.

Higher Order Moments of Proton Number Fluctuations in Au+Au Collisions at 1.23 AGeV studied with HADES

The HADES (High Acceptance DiElectron Spectrometer) experiment at the SIS18 accelerator of the GSI Helmholtzzentrum für Schwerionenforschung investigates heavy-ion collisions at kinetic beam energies of a few GeV per nucleon. We have carried out a detailed moment analysis of (net-)proton distributions of Au+Au collisions with 1.23A GeV (). We present here the correction techniques used in this investigation.

Measurements and understanding of fundamental properties of hot and dense nuclear matter with HADES

Heavy ion collisions at few GeV per nucleon energy range according to microscopic transport model calculations, create matter with densities several times larger that the ground density and heated to temperatures of 90 MeV. At such conditions the fundamental properties of the particles might be modified. Promising observables are the rare (multi-) strange hadrons and dileptons. The High Acceptance DiElectron Spectrometer was designed to measure rare and penetrating probes. It is a fixed target experiment located at SIS18 accelerator at GSI, Darmstadt, Germany. Recent results on dileptons and strangeness production in Au+Au collisions at center of mass energy are presented. Moreover, future experiments and upgrades of the apparatus are discussed.