Antiproton measurements in Ar+KCl reactions at 1.76 AGeV

A major part of the research in nuclear physics is concerned with the question of the properties of nuclear matter under extreme conditions. At the heavy-ion synchrotron SIS at the GSI, Darmstadt at bombarding energies between 1-2 AGeV nuclear matter is compressed to high densities of 2-3 times ρ0 and heated to temperatures of around 100 MeV. To keep the information of the early high density phase of the collision undistorted, particles should not undergo the strong interaction. Therefore HADES consequently reconstructs short lived resonances via their decay in e+e--pairs. A different approach is to investigate particles which are produced below their NN-threshold and therefore have a very steep excitation function, which makes them sensitive to predicted medium-effects. In this contribution we present a significant signal of the most extreme subthreshold produced particle at SIS energies, the antiproton. The free NN threshold of antiprotons corresponds to a kinetic beam energy of 5.6 AGeV, i.e. the threshold for antiprotons is much higher than achievable with the Fermi motion. The production yield is a promising observable for the properties of nuclear matter under these extreme conditions.