High Acceptance Dielectron Spectrometer (HADES) is a first FAIR experiment already existing and located at SIS18. The detector measures particles produced in proton, pion and heavy ion induced reactions on stationary targets made of various atoms (i.e. hydrogen, carbon, gold). The unique feature of the detector is an ability to measure virtual massive photons that converts in a very short time (Heisenberg principle) to electron-positron pairs. The beam particles are provided by SIS18 synchrotron which accelerates charged ions to kinetic energies of 1-2 GeV per nucleon. After completion of the physics programme at SIS18 the detector will be moved to CBM cave to continue measurements at higher beam energies provided by SIS100.
The mission of the experiments is twofold:
Using heavy ion collisions the experiment is studying properties of an exotic form of nuclear matter which is created in course of the reaction for an extremely short glimpse of time (~20 fm/c ≅ 1022s). This piece of matter, called fireball (see picture below), is characterized by a large density (exceeding even 2-3 times density of ordinary nuclear matter) and a temperature reaching up to 1022 Kelvin (kBT= 70 MeV)! The temperature is estimated from the spectral distribution of virtual photons (invariant mass of lepton pairs) measured in the detector. The distribution resembles the radiation emitted from a black body (Planck distribution). The fireball density, and also the temperature of the final stage of the collision (so called freeze-out), was also estimated from the spectra of emitted hadrons (mesons, protons) and showed to be slightly smaller. One should note that similar form of nuclear matter can be created on a macroscopic scale only in collisions of neutron stars.
The spectra of emitted lepton pairs carry-out also undistorted information of about masses of decaying mother particles and hence can be used to measure in-medium modification of hadron properties like masses and widths. Experiments provides striking evidences that hadrons do change their properties inside dense fireball medium.
Using proton-proton or pion-proton reactions the experiment probes internal structure of baryons measuring mass distribution of virtual photons emitted in transitions between excited and ground states of nucleons. From these measurements one concludes that nucleon looks like a composite object consisting from a quark core (made out of three valence quarks) and a meson “cloud” surrounding the core.
A group of physicist from the Department of Hadron Physics takes part in the HADES experiment since its foundation. The group provided several important components of the HADES detector developed in: the Gas detectors and the Electronics laboratories of the Cluster of Nuclear Physics Departments. The group takes also active part in the data analysis, specializing on electron-positron pairs and hadron production in proton and pion induced reactions. The HADES experiment is continuously supported by the Polish Science Foundation (NCN) by prestigious grants HARMONIA, HARMONIA and POLONEZ.
Further recommended reading :
- HADES Collaboration, "Probing dense baryon-rich matter with virtual photons”, Nat. Phys. 15, 1040–1045 (2019);
- P. Salabura (Jagielonian U.), J. Stroth (Frankfurt U., Inst. Kernphys. and Darmstadt, GSI and HIC for FAIR, Frankfurt), “Dilepton Radiation from Strongly Interacting Systems”, arXiv:2005.14589 [nucl-ex].
Contact: prof. dr hab. Piotr Salabura