Dilepton Production in Pb-Au Collisions at 158 GeV per Nucleon Using the CERES Detector

Abstract

This thesis deals with the measurements and results of inclusive e+e− pair production in ultra-relativistic Pb–Au collisions at 158 GeV per nucleon. The measurements were performed in the framework of the CERES experiment at the CERN SPS.

The field of ultra-relativistic heavy-ion collisions is particularly interesting in light of the prediction of QCD that at high temperatures and/or high baryon densities matter is expected to undergo a phase transition leading to a deconfined state – called quark-gluon plasma (QGP) – and to chiral symmetry restoration. Dileptons provide a unique probe of these predictions. Since they interact only electromagnetically, they have a long mean free path and therefore carry information about the fireball – earliest and most interesting stage of the collision. CERES is the only experiment dedicated to the measurement of low-mass (m < 1.5 GeV/c^2) electron-positron pairs and direct photons produced in ultra-relativistic heavy-ion collisions at the CERN SPS. In order to perform this interesting and challenging experiment CERES has developed a dedicated spectrometer. It covers the pseudorapidity region 2.05 < η < 2.65 with 2π azimuthal symmetry and accepts a very broad range of transversal momentum. The essential components of the CERES spectrometer are two Ring Imaging Cherenkov detectors (RICH) – one situated before, the other after a short solenoid which provides an azimuthal kick between the two RICH radiators. The coils shape the field lines such that the first RICH has practically zero field, and the second RICH has straight field lines pointing to the target. With this spectrometer, CERES has carried out over the last seven years a systematic research programme with p, S and Pb beams, including p–Be and p–Au collisions (1993) at a beam energy 450 GeV, S–Au collisions (1992) at 200 GeV per nucleon and Pb–Au collisions (1995-96) at 158 GeV per nucleon.

In 1994-95 the CERES spectrometer was considerably upgraded in order to cope with Pb-induced collisions. The main items of the upgrade were a doublet of silicon drift detectors (SiDC) located just after the target, and a multiwire proportional counter with pad readout (the pad chamber, PC) located behind the double RICH spectrometer.

This thesis deals with the upgrade and with the analysis of the 1995 and mainly the 1996 Pb–Au data. The emphasis of this work is in achieving the best accuracy in the measurements, both hardware-wise and software-wise. In the data analysis, a new approach was developed for the systematic study and suppression of the combinatorial background. The main result is that within the experimental errors, dominated by 25% systematic errors in the mass range 0.2 < m < 1.5 GeV/c^2, there is an enhancement by a factor of about 1.8 of the measured e+e− yield over the yield from the known hadronic sources. The enhancement is most pronounced in the mass range 0.2 < m < 0.7 GeV/c^2 and at low e+e− transverse momentum (below 500 MeV/c). Although the error bars are large, the data clearly indicate a non-linear dependence of the excess as a function of multiplicity.

JEL Classification: Z00