Electron-pair production in 158 AGeV/c Pb - Au collisions from CERES

The CERES experiment is devoted to the measurement of low-mass electron pairs in proton and ion-induced collisions at the Super Proton Synchrotron accelerator. The main goal of the CERES experiment is to investigate modifications of hadron properties in hot and dense nuclear matter. A significant enhancement of dilepton pair yield with respect to expectations based on hadron decays was observed in the mass range 0.25 m 0.7 GeV/c. This fascinating result can only be described by models invoking in-medium modifications of the meson. To distinguish between different theoretical explanations of the enhancement the experiment was upgraded in 1998 by the addition of a radial TPC to improve the mass resolution in the vector meson region. In this thesis, the analysis of the data sample taken in 2000 in 158 A GeV/c Pb-Au collisions with the upgraded experimental setup is presented. The observed enhancement of the pairs confirms previous results of CERES. The data, compared to the theoretical calculations based on dropping mass approach and in-medium spectral function modifications, favor the latter.