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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.

AbstractThe primary purpose of the ALICE experiment at the Large Hadron Collider is to study the properties of nuclear matter at extremely high temperatures and energy density produced by relativistic nucleus–nucleus collisions. During the Long Shutdown 2 (2019–2022), new detectors were incorporated into the ALICE setup, including the Fast Interaction Trigger (FIT). The FIT detector system consists of three sub-detectors based on different technologies: FDD, FT0 and FV0. In addition to online functionality, the FIT data are used offline for multiplicity, centrality, collision time, event-plane determination, vertex position, and veto for selection of diffractive and ultra-peripheral heavy-ion collisions. The mentioned above global observables are essential for event characterization and the study of nuclear matter properties. This material presents a preliminary overview of FIT’s performance in the extraction of global observables from pp and Pb–Pb collisions during the LHC Run 3.

The Fast Interaction Trigger (FIT)—a hybrid system of three ALICE forward detectors—has participated in each data-taking session since the beginning of LHC Run 3 in July 2022. FIT is essential for ALICE operation and provides the fastest online trigger, measures precision collision time, charged-particle multiplicity and centrality, and determines the online vertex position. It also serves as the primary online luminometer used by the LHC for beam levelling at ALICE.

As a result of the LHC injectors upgrade after the Long Shutdown (2019-2020), the expected Pb-Pb luminosity and collision rate during the so called Runs 3 and 4 will considerably exceed the design parameters for several of the key ALICE detectors systems including the forward trigger detectors. Fast Interaction Trigger (FIT) will be the primary forward trigger, luminosity, and collision time measurement detector. It will also determine multiplicity, centrality, and reaction plane of heavy ion collisions. FIT is expected to match and even exceed the functionality and performance currently secured by three ALICE sub-detectors: the time zero detector (T0), the VZERO system (V0), and the Forward Multiplicity Detector (FMD). FIT will consist of two arrays of Cherenkov radiators with MCP-PMT sensors and of a single, large-size scintillator ring. Because of the presence of the muon spectrometer, the placement of the FIT arrays will be asymmetric: ∼800 mm from the interaction point (IP) on the absorber side and ∼3200 mm from IP on the opposite side. The ongoing beam tests and Monte Carlo studies verify the physics performance and refine the geometry of the FIT arrays. The presentation gives a short description of FIT, triggers and readout requirement for the ALICE Upgrade, a summary of the performance, and the outcome of the simulations and beam tests.

. 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.

Prototype of the fast timing Cherenkov detector, applicable in high-energy collider experiments, has been developed basing on the modified Planacon XP85012 MCP-PMT and fused silica radiators. We present the reasons and description of the MCP-PMT modification, timing and amplitude characteristics of the prototype including the summary of the detector's response on particle hits at oblique angles and MCP-PMT performance at high illumination rates.

Main properties of the multi-anode microchannel plate photomultiplier to be used in a Cherenkov detector are discussed. The laboratory test results obtained using irradiation of the MCP-PMT photocathode by picosecond optical laser pulses with different intensities (from single photon regime to the PMT saturation conditions) are presented.

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.

Analysis of fast timing and trigger Cherenkov detector's design for its use in collider experiments is presented. Several specific requirements are taken into account - necessity of the radiator's placement as close to the beam pipe as possible along with the requirement of gapless (solid) radiator's design. Characteristics of the Cherenkov detector's laboratory prototype obtained using a pion beam at the CERN Proton Synchrotron are also presented, showing the possibility of obtaining sufficiently high geometrical efficiency along with good enough time resolution (50 ps sigma).

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.