QCD calculations of thermal photon and dilepton production

Electromagnetic probes (photons or lepton pairs) have production rates that are very sensitive to the temperature of the medium in which they are produced (the larger the temperature, the larger the rate) and are therefore mostly produced during the very early stages of a heavy ion collision. In addition to being produced early, they are weakly coupled to nuclear matter and have therefore a mean free path which is large compared to the typical size of the system produced in a collision. This enables them to escape from the system without any reinteraction. These two properties combined make electromagnetic signals very good probes of the state of the system very early after the initial impact. Roughly speaking, a heavy ion collision can be divided into several distinct stages, as illustrated on figure 1. Some photons are produced in the initial partonic collisions. These prompt