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Register a new userSearching for Quark Matter with Dileptons and Photons: from SPS to RHIC
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Itzhak Tserruya
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The heavy-ion programme at the CERN SPS, which started back in 86, has produced a wealth of very interesting and intriguing results in the quest for the quark-gluon plasma. The highlights of the programme on dilepton and direct photon measurements are reviewed emphasizing the most recent results obtained in Pb-Pb collisions at 158 A GeV. Prospects from RHIC are discussed.
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The study of dilepton and direct photon emission was one of the main topics of the experimental program at the SPS devoted to the search of signals for QGP formation. Three generations of experiments, Helios-3, NA38/NA50, CERES and NA60 measured e+e- or mu+mu- production in various colliding systems and at different energies. While lepton pair production in p-A collisions was found to be reasonably well described by the expected sources, all experiments observed in nuclear collisions an excess of the yield above the extrapolation from p-A. As a result of this joint experimental effort we have currently a large amount of information characterizing this excess: its mass spectrum over the full range from 0.2 GeV/c^2 up to the J/psi, its transverse momentum spectra including their mass dependence, its angular distributions, its dependence on collision centrality over the complete range etc. Putting together all this information leads to the conclusion that what we observe is the long-sought thermal radiation from the fireball.
Results on low-mass dileptons, covering the very broad energy range from the BEVALAC up to SPS are reviewed. The emphasis is on the open questions raised by the intriguing results obtained so far and the prospects for addressing them in the near future with the second generation of experiments, in particular HADES, NA60 and PHENIX.
Most recent PHENIX results on electromagnetic probes are presented including first preliminary results obtained with the Hadron Blind Detector (HBD) on e+e- invariant mass spectra from Au+Au collisions at sqrt(s_NN) = 200 GeV.
A review on experimental results for direct photon production in heavy ion reactions is given. A brief survey of early direct photon limits from SPS experiments is presented. The first measurement of direct photons in heavy ion reactions from the WA98 collaboration is discussed and compared to theoretical calculations. An outlook on the perspective of photon measurements at RHIC is given.
We study the production of photons and dileptons during the pre-equilibrium Glasma stage in heavy ion collisions and discuss the implications in light of the PHENIX data. We find that the measured distributions of such electromagnetic emissions, while having some features not well understood if hypothesized to entirely arise from a thermalized Quark-Gluon Plasma, have some qualitative features that might be described after including effects from a thermalizing Glasma. The shape and centrality dependence of the transverse momentum spectra of the so-called thermal photons are well described. The mass and transverse momentum dependence of intermediate mass dileptons also agree with our estimates. The low transverse momenta from which the excessive dileptons (in low to intermediate mass region) arise is suggestive of emissions from a Bose condensate. We also predict the centrality dependence of dilepton production. Uncertainties in the current approach and improvements in the future are discussed.
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