Thermal Dileptons from Hot and Dense Strongly Interacting Matter

The NA60 experiment at the CERN SPS has studied muon-pair production in 158A GeV In-In collisions. The unprecedented precision of the data has allowed to isolate a strong excess of pairs above the known sources in the whole invariant mass region 0.2<M<2.6 GeV. The (mostly) Planck-like shape of the mass spectra, exponential m_T spectra, zero polarization and the general agreement with thermal-model results allow for a consistent interpretation of the excess dimuons as thermal radiation from a randomized system. For M<1 GeV, the process pi+pi- -> rho -> mu+mu- dominates. The associated space-time averaged rho spectral function shows a nearly diverging width in approaching chiral symmetry restoration, but essentially no shift in mass. Some in-medium effects are also seen for the omega, but not for the phi. For M>1 GeV, the average temperature associated with the mass spectrum is about 200 MeV, considerably above T_c=170 MeV, implying a transition to dominantly partonic emission sources in this region. The transition itself is mirrored by a large jump-like drop in the inverse slope of the transverse mass spectra around 1 GeV.