The
QCD vacuum is characterized by a complex structure of quark and gluon
condensates which are believed to cause the fundamental phenomena of
mass generation and confinement. In particular, the quark anti-quark
condensate spontaneously breaks the chiral symmetry of QCD and shapes
the spectrum of light hadrons. In high-temperature QCD matter the
quark condensate is expected to melt, leading to the restoration of
chiral symmetry. A long-standing question is how the melting of the
condensate affects the hadronic spectrum in matter, and how these
changes can be observed in experiment. The spectra of thermal dilepton
radiation in high-energy heavy-ion collisions are among the most
promising signals for the discovery of chiral restoration. We discuss
the current status in the theoretical description of the measured
dilepton spectra and the implications for the hadron-to-quark
transition in QCD matter.
Nuclear
Physics Colloquium