Hades

The HADES (High Acceptance Di Electron Spectrometer) system is a a fixed target experiment located at the GSI (http://www.gsi.de) facility near Darmstadt, Germany. It has been designed for electron-positron pair spectroscopy in proton and heavy ion induced reactions. The experiment is taking data since 2002. After a detector upgrade further experimental runs are beeing planned for the next years. HADES is an international collaboration with members from 17 institutions from 9 European countries.

Our group is involved in the upgrade of the trigger electronics, which is needed to select online the interesting events containing electron pairs.

Experimental Summary

The investigation of hadron properties inside nuclear matter at normal and high densities and temperatures is one of the main goals of current nuclear physics studies. Hadron induced reactions on heavy nuclei (/e.g. /Au, Pb) are the proper tool to probe particle properties in long-living ground state nuclear matter. Heavy ion collisions at energies of 1-2 AGeV can be used to create a reaction region of increased density for as long as 10 fm/c. Under these conditions, considerable modifications of basic hadron properties (masses, decay widths, etc.) are expected and probably can be verified for the first time experimentally by high resolution lepton pair decay measurements. A more detailed physics motivation is given below.

Why electron-positron pair spectroscopy?

Electrons do interact by the strong force (like for example pions) and can therefore leave the reaction fireball without final state interaction. This means, the original energy and momentum of the particles is preserved. The mass and width of the meson can be reconstructed from the four-momentum of its decay products. A backdraft of this method is the low branching fraction in the order of 10^-4. This means only every tenthousandth meson produces in its decay an e+e- pair. Therefore a very selective trigger system is needed to store the interesting events.

Physic motivation: Do hadrons change their mass inside nuclear matter?

A few indications exist that something new happens in dense hadronic matter: by studying the mass spectrum of e+e- pairs emitted from the collision of two nuclei, a clear overproduction in the region below the Rho and Omega vector mesons has been observed by previous experiments. Does this mean that these mesons change their mass inside dense nuclear matter? Is it a shift or a broadening? Or both? Is this an indication of chiral symmetry restoration? The signal is not clean enough, the statistics are quite poor. Theoreticians have started producing lots of predictions, quite often controversial, and therefore data is needed.