The High Acceptance Di Electron Spectrometer (HADES) is a detector system for lepton pair spectroscopy built up at GSI (Darmstadt) by a European collaboration involving groups from 19 institutions in 9 countries. Initial experiments are planned for November 2000. The physics program of HADES is broad and includes the study of electron-positron pair emission in relativistic heavy ion collisions, di-lepton production in elementary reactions and experiments aimed at studying the structure of hadrons. With the available beam energies (4.5 GeV protons, 1-2 AGeV heavy ions) the interest is focussed on lepton pair invariant masses up to 1 GeV/c2.
As leptons do not undergo strong interactions but interact only electromagnetically they are considered penetrating probes of compressed hadronic matter formed within the collision zone of relativistic heavy ion reactions. Predominant dilepton sources for me+e- > 500 MeV/c2 are direct or Dalitz decays of the light vector mesons , and produced in these collisions. The very weak final state interaction allows a measurement of their masses resp. spectral functions in the hot and compressed hadronic environment by reconstruction of the invariant mass from the lepton momenta. This provides the possibility to test theoretical predictions for a partial restoration of chiral symmetry at elevated temperatures and baryon densities which - as an experimentally observable consequence - would lead to reduced meson masses. The HADES detector is specifically designed to provide an excellent mass resolution (m/m ~1%) and a very large acceptance for comprehensive studies of the behaviour of , and - mesons in the nuclear medium. With a lifetime of ~1 fm/c the -meson is ideally suited for probing vector meson properties in the transient high density phase (~10 fm/c) of heavy ion reactions. Possible modifications in mass and decay width at normal matter density - predicted within the same theoretical frame work as well - can be tested with the - (~22 fm/c) and - mesons (~44 fm/c) which may decay to a considerable fraction within the target nucleus if produced almost recoil-free using a -beam.
These series of experiments, exploiting the full range of primary and secondary beams available at GSI, is expected to make important contributions to our understanding of Quantum Chromodynamics in the non-perturbative regime and, in particular, will provide information on the origin of hadron masses.
Below 500 MeV/c2 dilepton spectra in heavy ion collisions are dominated by -Dalitz decays, -decays and nucleon-nucleon as well as -nucleon bremsstrahlung. For a quantitative interpretation of dilepton spectra in heavy ion collisions the latter contributions have to be measured separately in elementary p- and -induced reactions. The -Dalitz decay contribution can be identified by measuring e+e- coincidences employing an additional photon detector (e.g. TAPS).
The electromagnetic structure of mesons and baryons can be studied by measuring their time-like formfactors which are important ingredients in any modelling of hadrons. Transition form factors can be deduced from the shape of the dilepton invariant mass spectra from e+e- or e+e- Dalitz decays of neutral mesons. A precise knowledge of these formfactors is also important for a quantitative understanding of dilepton spectra from heavy-ion and -induced reactions. To perform such measurements with HADES the present set-up may be combined with an electromagnetic calorimeter for photon detection.
This brief summary illustrates the broad physics potential of HADES which ranges from probing compressed hadronic matter in heavy-ion reactions and the study of chiral symmetry restoration to elementary properties of hadrons.
Authors: J.F., V.M.