Lepton pair production in relativistic heavy ion collisions at CERN SPS energies

Abstract

The production of lepton pairs as probes for the nuclear collision dynamics at CERN SPS energies is discussed in some detail. We review the present theoretical status of different dilepton sources. Most probably thermal dileptons from the quark gluon plasma, if such a state is formed at all in present CERN heavy ion collision experiments, are covered under a huge background of lepton pairs from meson decays at low invariant masses and Drell-Yan pairs at higher invariant masses. However, dileptons, and in particular muon pairs look promising as probes for the hadron dynamics of the nuclear collision process. We emphasize the implications of non-equilibrium pion dynamics for the production of lepton pairs. We find a strong enhancement in the low invariant mass region, 2 m π ≤ M μ + μ − ≤ 600 MeV, for lepton pairs which are produced via annihilation of pions in the hot and dense collision zone. Various sources of background which could mask this enhancement are discussed in some detail. We present and compare calculations for dilepton mass spectra based on different nuclear collision scenarios proposed so far. Finally we show that the dilepton yields from the low mass vector mesons contain valuable information on the lifetime of the hadronic phase.