  Employing thermal dilepton rates based on medium-modified electromagnetic
correlation functions 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
\rho-meson spectral functions, as following from hadronic many-body theory,
provide an accurate description of the data up to dimuon invariant masses of
about M\simeq 0.9 GeV, with good sensitivity to details of the predicted
\rho-meson line shape. This, in particular, identifies baryon-induced effects
as the prevalent ones. We show that a reliable description of the \rho
contribution opens the possibility to study further medium effects: at higher
masses (M \simeq 0.9-1.5 GeV) 4-pion type annihilation is required to account
for the experimentally observed excess indicating precursor effects of chiral
symmetry restoration (``chiral mixing''), while remaining structures in the
\omega and \phi region are suggestive for modifications in their line shapes as
well.

High Energy Physics - Phenomenology