Rare hadronic processes
in nucleus matter

The aim of this research project is to contribute to the phenomenon "mass" of particles. The intention is to detect in particular vector mesons in nuclear matter through direct electromagnetic probes in the HADES system. A shift or broadening of the energy-dependent excitation strength can be interpreted as modification of the mass of the vector mesons. Vector mesons are dipole excitations of the quantum-chromodynamic ground state. Analogous to the Zeeman effect, where excitation spectra of atoms are changed within an external magnetic field, a change of the dipol excitations (vector mesons) in the field of the strong interaction, which is produced by surrounding nuclear matter, is expected.

The main idea of the HADES project is the identification of vector mesons (V = ρ, ω and Φ) through their electromagnetic decays into virtual photons (electron-positron pairs e-+e+). There is minor interaction of the electrons and positrons with the surrounding nuclear matter. Thus, they carry unaltered information on the spectral distribution of vector mesons decaying within atomic nuclei. The electromagnetic decays V-> e++e- are rare processes to be selected out of the dataflow by a complicated trigger system.

HADES was built by an international collaboration at the heavy ion synchrotron at GSI Darmstadt. The participation in this collaboration with about 100 scientists gives rise to an intensive knowledge transfer in particle physics (Monte-Carlo simulations, detector construction, fast front-end electronics, data analysis) from which the HZDR and in particular ELBE are strongly benefiting. The technical contribution of the ISP to this worldwide unique spectrometer consists of the construction and operation of detectors (Multi-wire Drift Chambers - MDC) and data evaluation, which is accomblished by doctorands, financed primarily by project fundings.

These evaluations are accompanied by theoretical analyses to find out which medium modifications of the vector mesons are necessery in order to describe the data. For this purpose, theoreticians of the institute developed QCD sum rules, effective hadron models and transport codes.

HADES covers three fields of work: (i) heavy ion experiments with nuclei of lower mass numbers, (ii) elementary reactions on the proton and deuteron with proton and pion beams, and (iii) heavy ion reactions with nuclei of higher mass numbers. The primary goal of HADES are precision measurements of e+e- pairs, with the aim to identify medium modifications of light vector mesons. For this purpose the analysed data has to be compared with complex models.