Dilepton production in ultrarelativistic heavy-ion collisions.

Many probes have been proposed to map out the behaviour of hot and dense hadronic matter and also to highlight its eventual transition to a plasma of quarks and gluons. It is outside the scope of this talk to review them all or even partially: we will concentrate on electromagnetic radiation and more specifically on the emission of thermal lepton pairs. Those suffer minimal final state interactions and will thus carry valuable information about their emission site. Lepton pairs should then tell us about in-medium temperatures and densities and will constitute information that is complementary to that carried by hadronic observables. The rate for dilepton emission can be expressed in terms of the finite-temperature retarded photon self-energy :