[en] In this thesis we study dilepton production in Pb-Au collisions at 158 GeV/c per nucleon measured with the Cherenkov Ring Electron Spectrometer (CERES) at the Super Proton Synchrotron (SPS) at CERN. The data taken in the year 2000 represent the first CERES runs with the fully operational Time Projection Chamber (TPC). The upgrade of the spectrometer with this powerful tracking device allows to realize a new approach for particle identification with the Ring Imaging Cherenkov (RICH) detector. We present the development and optimization of track-based ring reconstruction methods which allow to identify both electrons and pions at momenta above the Cherenkov threshold. To compare the performance of different methods, the reconstruction is applied to the CERES data. This allows to determine experimentally, for the first time, the CERES RICH efficiency and background rejection power. The new particle identification is used for low-mass dielectron reconstruction. Significant improvements in the simulation of the detector allow to evaluate the electron pair reconstruction efficiency with high accuracy and to study the origin of electron pair combinatorial background. The results motivate an extension of the TPC tracking, to improve the reconstruction efficiency for low-momentum tracks, and allow better identification of electron background. Our results corroborate the previous CERES findings of an enhancement of electron pairs over the hadronic cocktail in the invariant mass region m_inv >or similar 0.2 GeV/c². Comparison to previous analyses of the same data shows good consistency and supports our estimate of the systematic uncertainties of the electron analysis. Our results supply evidence for a modification of the spectral function of the ρ meson in the hot and dense medium and strongly support baryon-induced interactions at the origin of the effect. Furthermore, we present in this work the first direct measurements of transition radiation (TR) spectra in irregular radiators, carried out with prototypes of the ALICE (A Large Ion Collider Experiment) transition radiation detector (TRD). The measurements are confronted with calculations for regular radiators. The TR yield and the shape of the spectral distributions for different electron momenta are well described by the simulations. (orig.)