The High Acceptance Di-Electron Spectrometer (HADES) is designed specifically to detect electrons and positrons from heavy ion and hadron (pion, proton) collisions. In our analysis, these single leptons are identified using state-of-the-art techniques involving artificial intelligence and machine learning. Leptons are matched into respective dilepton pairs. They can be investigated in a variety of ways but a major focus is set to reconstruct the invariant mass spectra. Dilepton invariant-mass spectra are the only observable which gives direct access to the in-medium modification of hadronic spectral functions. Using simulations as well as information about other measured particles, e.g. pions, one can further correct and isolate the invariant mass spectrum to only represent dileptons coming from the hottest and densest phase of the collision. The significant excess radiation of dileptons, beyond final-state hadron decays, can be seen in Figure 2. It can be isolated by subtracting the hadronic cocktail from the data. The resulting spectrum shown in Fig.3 allows to extract important physics: microscopic properties of the hot and dense medium, its life time and temperature, its collective expansion dynamics or its transport properties.
We are analysing dielectron data from heavy-ion collisions (Au+Au, Ag+Ag) as well as from elementary collisions (proton, pion beams impinging on proton or heavy target).
The Relativistic Heavy Ion Collider provides energy coverage spanning from √(sNN) = 7.7 – 200 GeV. The STAR detector (for Solenoidal Tracker at RHIC) has been successfully upgraded in 2018 – 2019. Our group is participated in one of the major upgrades to the STAR detector namly the End-cap Time-Of-Flight (eTOF) system (in cooperation with CBM, so called FAIR Phase 0 project). The eTOF upgrade will provide particle identification in the extended pseudo-rapidity range provided by the inner TPC sector upgrade.
We are analysing dielectron data and aim to extract the temperature of strong-interaction matter formed in Au-Au collisions at 200 GeV from data collected by STAR at RHIC in 2014 and 2016.
In 2018, utilizing the gold fixed-target installed in the STAR experiment, we recorded 260 million Au-Au events at a collision energy as low as √(sNN)= 3 GeV and in 2021 collected additional 2 billion events, thus enabling studies of thermal dilepton spectra.
FAIR – Facility for Antiproton and Ion Research in Europe GmbH, with its SIS100 accelerator, will be unique in the exploration of the QCD phase diagram in the region of high net-baryon densities by means of rare and penetrating probes. High-rate operation is the key prerequisite for high-precision measurements of multi-differential observables and of rare diagnostic probes which are sensitive to the dense phase of the nuclear fireball. The CBM experiment will perform pioneering multi-differential measurements of lepton pairs over the whole range of invariant masses emitted from a hot and dense fireball. A very important part of the CBM research program will be the high-precision measurement of the dilepton invariant mass distribution between 1 and 2.5 GeV/c2 for different beam energies. With respect to top SPS, RHIC and LHC energies, the contribution of dileptons from Drell-Yan processes or correlated charm decays or even QGP, which also populate this mass region, are dramatically reduced at a beam energy of 10A GeV. This allows direct access to the early fireball temperature. The precise measurement of the energy dependence of the spectral slope opens the unique possibility to measure the caloric curve which would be the first direct experimental signature for phase coexistence in high-density nuclear matter. Our group is working on the systematic investigation of the multi-differential emission probability of dileptons using realistic event generators and realistic detector responses. The sensitivity of the respective experiments is evaluated by means of Monte Carlo simulations.
The coupling of virtual massive photons to baryon resonances can experimentally be probed by means of the πN→R→e+e-N process for which neither experimental data nor reliable theoretical predictions exist. In view of these uncertainties in the theoretical description of the elementary cross section, it is necessary that the exclusive cross sections for dilepton production on the nucleon are measured. Our group also studies pion and proton induced reactions.
The understanding of the dilepton excess radiation calls for supporting studies based on various model calculations. Comprehensive information on meson production is thereby mandatory to benchmark and constrain those calculations. In this context the neutral π and η mesons are of particular interest as they contribute largely to the dilepton spectrum via their Dalitz decays. The measurement of the electromagnetic decays of π0 and η is possible via external conversion of photons in detector material or by detecting photons in the electromagnetic calorimeter. Our group is performing reconstruction of π0- and η-mesons.
In heavy-ion collisions, the overall motion of the particles manifests in a phenomenon known as collective flow, but also in the vortical structure of the system. In the fluid-like matter, the vorticity transforms into the spin polarization of the produced particles. The most prominent effect is the global polarization due to the finite impact parameter in non-central collisions. The parity violation in weak decays allows to measure the spin direction of e.g. strange hyperons. The analysis requires large statistics and a pure sample, such that machine learning techniques are used to improve the selection efficiency. To date, the largest global polarization in heavy-ion collisions has been measured with HADES, making this field of research very interesting.
Significant progress in the understanding of heavy-ion collision could be achieved if the dynamics of excited baryons would be known better. The direct reconstruction of these particles is challenging as the resonances are broad and the extraction of cross section is determined by the knowledge of the underlying combinatorial background. Our group has developed an iterative method which identifies signal and background contributions without input models for normalization constants is presented. See for details our publication Eur.Phys.J.A 55 (2019) 11, 204 • e-Print: 1808.05466 [physics.data-an]
Using this innovative method we reconstructed correlated pion-proton pair emission off hot and dense QCD matter (HADES Collaboration, Phys.Lett.B (2021) 136421 • e-Print: 2012.01351 [nucl-ex]).
