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Bochum 1998 – scientific programme

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HK: Hadronen und Kerne

HK 65: Heavy Ion Reactions VIII, Ultrarelativistic Energies

HK 65.1: Talk

Thursday, March 19, 1998, 14:00–14:15, F

Low-mass dilepton pairs from 160 AGeV Pb-Au collisions — •C. Voigt for the CERES Collaboration — Physikalisches Institut der Universität Heidelberg, Philosophenweg 12, D-69120 Heidelberg

The observation of a low-mass dilepton enhancement in S-induced collisions at the CERN SPS stimulated an intense discussion of the way vector mesons change in dense and hot hadronic matter. Dropping meson masses, derived from theoretical arguments based on chiral symmetry restoration and more conventional approaches like collision broadening of the resonances create sufficient strength to explain the data. In view of the highly complex physics processes involved more and better data is mandatory to draw definite conclusions.
We report a measurement of low-mass electron pairs observed in 160 GeV/nucleon Pb-Au collisions. The data was taken with the CERES/NA45 spectrometer during the 1995 and 1996 lead beam runs at the CERN SPS. Above masses of 200 MeV/c2 the e+e pair continuum is enhanced by a factor of 2-3 compared to the yield expected from neutral meson decays by scaling from p-induced reactions. The enhancement is most pronounced in the invariant mass region from 300 to 700 MeV/c2. The statistics available enables us to study the impact-parameter dependence of the enhancement. We find evidence for a strong increase of the pair excess with centrality as expected for thermal radiation. First data on p spectra show that the yield is amplified over the entire range, but it is strongest at the lowest p measured.

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