kaempfer Annual Report 2000

Analysis of Dilepton Data from Heavy-Ion Experiments at CERN-SPS ^B,W

K. Gallmeister, B. Kämpfer, O.P. Pavlenko¹, C. Gale²

An excess of dileptons in lead-beam induced heavy-ion collisions has been reported (i) by the CERES collaboration in the low-mass region of dielectrons and (ii) by the NA50 collaboration in the intermediate-mass region of dimuons. This excess points to the existence of a system of thermalized matter emitting dileptons [1].
We achieve a good description of the mentioned data by a superposition of the thermal yield and either the hadronic cocktail in case of CERES data or both the contributions from correlated semileptonic decays of open charm mesons and the Drell-Yan process for NA50 data (see figs. 1 and 2 and [1] for details).
The thermal source is characterized by a mean temperature áTñ = 170 MeV. A similar value is found in an analysis of the dilepton data of the CERES, Helios/3 and NA38 collaborations with the sulfur beam [2]. In the dynamic scenario we find an initial temperature of the thermal system in the order of 200 MeV, i.e. a value larger than the expected deconfinement temperature.

Fig. 1 A comparison of our model with the CERES data. The dashed curves are the hadronic cocktail published by the CERES collaboration, the solid curves are the contributions from the thermal system. The sum is depicted by the gray curves. Left panel: the invariant mass spectrum, right panel: the transverse momentum spectrum for the invariant mass region M = 0.25...0.68 GeV.

Fig. 2 The data of the NA50 collaboration for the invariant mass spectrum (left) and the transverse momentum spectrum in the invariant mass region M = 1.5...2.5 GeV (right). The dashed curves are the Drell-Yan contributions, the dotted curves show the contributions of open charm and the solid lines depict the yield of the thermal system. The sum is shown by the gray curves.

¹Institute for Theoretical Physics, 252143 Kiev - 143, Ukraine
² McGill University, Montreal, QC, H3A 2T8, Canada

References

[1] K. Gallmeister, B. Kämpfer, O.P. Pavlenko, Phys. Lett. B 473, 20 (2000)