Dileptons at Colliders as Probes of the Quark–Gluon Plasma

Ultrarelativistic heavy-ion collisions are used to create a deconfined state of quarks and gluons, the quark–gluon plasma (QGP), similar to the matter in the early Universe. Dileptons are a unique probe of the QGP. Being emitted during all stages of the collision without interacting strongly with the surrounding matter, they carry undistorted information about the medium evolution. The mass of the lepton–antilepton pair gives a unique means to separate partonic from hadronic radiation. Thus, dileptons can be used to study the QGP equilibration time, its average temperature, and effects related to the restoration of chiral symmetry in the hot medium via vector meson decays. This information is not accessible with hadrons. The price to pay is a large background from ordinary hadron decays. We summarize the potential of dilepton measurements, the results obtained so far at colliders, and the ongoing efforts regarding future experiments with further increased sensitivity.