Study of electron pair and photon production in lead-gold collisions

This is a continuation of the NA45 experiment dedicated to the measurement of electron-positron pairs and direct photons produced in Pb-Pb collisions at SPS energies. The main goal remains as outlined in NA45. The strong enhancement of low-mass pairs, over the expected yield from hadronic sources, observed in S-Au collisions by NA45, adds considerably to the physics potential and to the interest in the measurement of these variables. \\\\The figure shows the layout of the CERES spectrometer which has been upgraded to cope with the higher multiplicities and background of central Pb-Pb collisions. The basic spectrometer remains unchanged, namely two Ring Imaging Cherenkov detectors (RICH), one situated before the other after a short superconducting double solenoid. The main elements of the upgrade are additional detectors, two silicon radial-drift chambers (instead of one in the original set-up) and a pad chamber (a large MWPC with pad readout) located behind the spectrometer. They allow real tracking and help the pattern recognition of the RICH detectors by providing an $a$ $priori$ knowledge of the ring location in the UV counter. The two silicon radial-drift chambers allow precise vertex reconstruction and provide rapidity density information. They also play, together with the first RICH, a major role in the rejection of $\gamma$ conversions and $\pi^{0}$- Dalitz decays. The upgrade includes also a new DAQ system with a rate capability higher by more than one order of magnitude compared to the one used in NA45.