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HK: Hadronen und Kerne
HK 65: Heavy Ion Reactions VIII, Ultrarelativistic Energies
HK 65.1: Vortrag
Donnerstag, 19. März 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.