Dileptons

Dileptons are pairs of leptons produced in high-energy particle interactions. The most commonly studied channels are dielectron (e+ e−) and dimuon (μ+ μ−) pairs, but any two leptons of opposite charge and possibly different flavours can form a dilepton system. Dileptons are produced when a virtual photon or a Z boson, and at lower masses often a vector meson, decays into a lepton pair.

Production mechanisms include Drell–Yan processes in hadron collisions, where a quark and antiquark annihilate into a virtual photon or Z that decays to l+ l−; decays of heavy-flavor hadrons (from semileptonic decays of charm and bottom) producing correlated leptons; and decays of light vector mesons such as rho, omega, phi into lepton pairs. In ultrarelativistic heavy-ion collisions, dileptons also arise from the thermal radiation of a hot, strongly interacting medium (quark-gluon plasma), providing a penetrating probe of early-time dynamics.

Because leptons interact only weakly, dileptons traverse surrounding matter with little final-state interaction, carrying information about the conditions at their production. The invariant mass spectrum of dileptons serves to study electroweak processes (e.g., Z boson production) and hadronic structure, as well as to explore in-medium modifications of vector mesons and the properties of the quark-gluon plasma in heavy-ion physics. Experiments measure differential yields as a function of invariant mass, pair transverse momentum, and angular distributions, and compare dielectron and dimuon channels to test lepton universality and detector performance.

Dileptons have historically contributed to tests of the Standard Model and searches for new phenomena, and remain a sensitive probe in collider, fixed-target, and heavy-ion experiments worldwide.