Comprehensive Interpretation of Thermal Dileptons at the SPS

Employing thermal dilepton rates based on a medium-modified electromagnetic correlation function we show that recent dimuon spectra of the NA60 collaboration in central In-In collisions at the CERN-SPS can be understood in terms of radiation from a hot and dense hadronic medium. Earlier calculated in-medium ρ-meson spectral functions provide an accurate description of the data up to dimuon invariant masses of about M≃0.9 GeV, with good sensitivity to the predicted ρ-meson line shape, identifying baryon-induced modifications as the prevalent ones. A reliable evaluation of the ρ contribution enables the study of further medium effects: at masses M>0.9 GeV, 4-pion type annihilation accounts for the experimentally observed excess (possibly augmented by effects of “chiral mixing”), while predictions for thermal emission from in-medium ω and φ mesons may be tested in the future.