Study of electron pair production in hadron and nuclear collisions at the CERN SPS

The NA45/CERES experiment investigates primarily the production of electron-positron pairs and of direct photons in proton-nucleus and nucleus-nucleus collisions. For electron-positron pairs the experiment studies the continuum in the mass region of about 0.05 to 2 GeV/c$^2$ and the vector mesons $\varrho ,~ \omega$, and, $\phi$. Since for electromagnetic probes final state interactions are practically negligible these observables are unique for studying the evolution and dynamics of ultrarelativistic heavy-ion collisions from the hot and dense early stage where a quark-gluon plasma is expected to be formed to the final freeze-out stage when hadrons decouple.\\ \\ The experiment also studies the spectral distributions of charged particles, their distribution relative to the reaction plane, and identified high momentum pions. Another topic of investigation are QED pairs produced in peripheral nuclear collisions.\\ \\ The first phase of the experiment, NA45, has been concluded with two main results: i) There is a significant excess, by a factor of about 5, of dielectron pairs in the mass range between 0.2 and 1 GeV/c$^2$ in S-Au collisions at central rapidities as compared to the hadronic background established in $p$-Be and $p$-Au collisions. ii) The measurement of photons in S-Au collisions by the conversion method provides an upper limit (90 \% CL) of 7\% for direct photons relative to photons from hadron decays.\\ \\ For the Pb beam program the experiment is upgraded in steps (NA45/2). In a first stage the multiplicity- and rate-capability where increased and the experiment found that i) also in Pb-Au collisions there is an enhancement in the number of dielectron pairs comparable to what was found with the S-beam and ii) the enhancement grows much steeper than linearly with the charged particle multiplicity in the event. Driven by these results the experiment is upgraded further by the addition of a TPC to improve the momentum resolution.\\ \\ The experimental set-up consists of a double spectrometer covering a region near mid-rapidity ($2.1 < \eta < 2.7$) with full azimuthal coverage. A set of silicon drift chambers (SiDC1/2) is used to reconstruct the vertex and to measure the polar and azimuthal angles of each produced particle. Electrons are identified in two Ring Imaging Cherenkov detectors (RICH). They operate at a threshold of $\gamma$=32 and are essentially blind to the produced hadrons. In a pad chamber downstream of the second RICH another spacepoint is measured for every particle. A first momentum measurement is derived from the deflection in azimuth in a magnetic field generated by two superconducting coils (main coils) as indicated by the dashed lines in the figure. In the upgrade, the pad chamber is replaced by a TPC continously tracking particles over 2m in a magnetic field generated by two large coils (TPC coils), again with opposite current directions to deflect particles mostly in azimuth. The aim is to achieve a mass resolution in the $\varrho, \omega, \phi$ region of 2\%.