Low mass e"+e"- pair production as function of multiplicity in nucleus-nucleus collisions at 200 GeV/u Final scientific report

During the grant period, CERES has completed its original physics programme with p, S and Pb beams, including the measurements of electron pairs in p-Be (a very good approximation to p-p collisions) and p-Au collisions at 450 GeV/c, S-Au collisions at 200 GeV/nucleon and Pb-Au collisions at 158 GeV/nucleon. This systematic approach has revealed a very interesting result - the observation of a strong enhancement, over the known hadronic sources, of low-mass electron pairs in S-Au and Pb-Au collisions, suggesting the onset of a new source, beyond the mere superposition of p-p collisions. This result has triggered a huge theoretical activity mainly stimulated by the interpretations which invoke in-medium modifications of the vector mesons and in particular a decrease of the #rho#-meson mass as a precursor of chiral symmetry restoration. The possibility that first hints of chiral symmetry restoration might have been observed, have prompted us to engage in a major upgrade of the CERES apparatus to considerable improve its mass resolution. The main goal is to measure directly the yield and properties (mass and width) of the vector mesons thereby seeking compelling evidence of the onset of chiral symmetry restoration. (orig.)SIGLEAvailable from TIB Hannover: DtF QN1(65,15) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekGerman-Israeli Foundation for Scientific Research and Development (GIF), Oberschleissheim (Germany)DEGerman