Low-mass dielectron production in Pb - Au collisions at 158 AGeV using the upgraded CERES spectrometer

This thesis deals with the measurements and results of inclusive $e^{+}e^{-}$ pair production in ultra-relativistic Pb - Au collisions at 158 AGeV . The measurements were performed in the framework of the CERES experiment at the CERN SPS. The main aim for studying ultra-relativistic collisions of heavy ions is to investigate the behavior of nuclear matter under extreme conditions of density and temperature. Lattice QCD calculations predict that under conditions of suciently high temperature and/or baryon density the quark and gluon degrees of freedom, usually conned within hadrons, are liberated. This state of matter is called the Quark-Gluon Plasma (QGP). Another phenomenon associated with the QGP phase transition is the restoration of the spontaneously broken chiral symmetry. Lepton pairs are unique probes of the initial reball. Since they interact only electromagnetically, they have a long mean free path and can leave the interaction region without any additional interactions carrying direct information on the medium. In addition, dileptons allow the study of the leptonic decays of the $\rho, \omega$ and $\phi$ mesons. The $\rho$ meson is of particular interest, since its lifetime is very short and almost all $\rho$ mesons decay inside the fireball. This makes the $\rho$ meson one of the best potential signals of chiral symmetry restoration. CERES is the only experiment specically designed and dedicated to the measurement of low mass electron-positron pairs at the CERN SPS. CERES has carried out a systematic programme including the measurement of dilepton pairs with $p$, S and Pb beams on a Au target. The essential elements of the original CERES spectrometer are two Ring Imaging Cherenkov (RICH) detectors covering the pseudorapidity region of $2.1 \leq \eta \leq 2.7$ with full azimuthal symmetry. A doublet of Silicon Drift Chambers (SiDC) is located before RICH-1 providing a very precise measurement of the event vertex position and the event multiplicity. In the year 1998 the CERES detector was upgraded with the addition of a TPC (Time Projection Chamber) in order to achieve a better mass resolution and to improve the rejection of $\pi^{0}$ Dalitz and $\gamma$ conversion dilepton pairs. The TPC is placed right after the mirror of RICH-2, and it is operated within a magnetic field generated by two coils with opposite currents. The radial component of the field detects the charged particles in the $\varphi$ direction, which allows us to calculate the momentum of charged particles. This thesis deals with the upgrade and the analysis of the 2000 Pb 􀀀 Au data. The thesis describes a new approach which was developed for the pair analysis and background rejection for the upgraded CERES spectrometer. 17.5 million events with a 7% centrality selection were processed. 3471 126 pairs for masses m < 0:2 GeV=c2 with S=B ratio of 1/1.8 and 1747 258 pairs for masses m > 0:2 GeV=c2 with S=B ratio of 1/18.6 were reconstructed. For masses m < 0:2 GeV=c2, the invariant mass spectrum is in agreement with the expectation from hadron decays. For masses m > 0:2 GeV=c2, there is a clear enhancement. Most of the enhancement is observed around the mass interval of 0:3 􀀀 0:6 GeV=c2 and in the soft pair pT region (pee T < 0:5 GeV=c2). The enhancement factor, dened as the measured e+e􀀀 yield over the calculated yield from the known hadronic sources for masses $0.2 < m < 1.6$ GeV/c$^{2}$, is 2.90 $\pm$ 0.32(stat) $\pm$ 0.44(syst) $\pm$ 0.87(decays) (where the last term represents the systematic errors in the calculated yield). The data show a non-linear dependence of the enhancement as a function of centrality. The theoretical explanations of the present results are discussed. The enhancement of dilepton pairs is interpreted as direct evidence of thermal radiation dominated by the $\phi^{+}\phi{-}$ annihilation via a modified $\rho$ spectral function. The data favor broadening of the $\rho$ meson shape rather than a drop of its mass.