Dilepton
With Dilepton is called a correlated pair of a Lepton® and a antilepton, its anti-particles , which in the nucleus collisions in particle accelerators is generated -Experimenten. Dileptons arise from electromagnetic decay or annihilation processes . Dileptons are either dielectrons ( electron- anti- electron pairs) or dimyons ( muon- anti-electron pairs). Tau-lepton pairs are usually not referred to as dileptons because they are not measured because of their high mass and short lifetime.
Since (electrically charged) leptons are only subject to the electromagnetic and weak interaction , they are not deflected by the strongly interacting medium that is created by the collisions. Dileptons are therefore used as probes for the investigation of properties of the medium and of the particles in the medium that decay into dileptons. There is particular interest in the investigation of the quark-gluon plasma and the restoration of the chiral symmetry .
production
There are several sources of dilepton emission in a nuclear collision.
In initial collisions of two nucleons of the colliding nuclei, a quark of one nucleon anihilate with a quark of another nucleon. This process results in a virtual photon, which then decays into a dilepton ( Drell-Yan process ).
During the collision, resulting Charm- or bottom quark -Antiquark pairs can subsequently with lighter quark a D- or mesons B -pair ( , ) form. The mesons then decay in a semileptontic decay . The decays of the two meson partners together produce a dilepton.
Another source is the annihilation of quark-antiquark pairs ( ) from the quark-gluon plasma that is created during the collision and in which quarks are not bound in hadrons. This contribution to the measurement is often dominated by other contributions and is therefore difficult to access experimentally. The measurement of this contribution is one of the few measurements that allows direct conclusions to be drawn about the temperature of the quark-gluon medium.
Dileptons are also produced by hadronic processes. One example is the collision of a positive pi meson with its negative antiparticle, which creates a lepton pair. In addition, dileptons are produced in resonance decays. Some resonances (e.g. vector mesons such as ρ-meson , ω-meson , φ-meson or J / ψ-meson ) decay directly into a lepton pair. This carries the same quadruple impulse and thereby allows direct access to properties such as the mass of the decaying resonance. The measurement of resonance properties is of particular interest because this can theoretically be linked to the restoration of chiral symmetry. This change in symmetry, like the contribution, could indicate the formation of the quark-gluon plasma.