An upsilon particle makes its mark

10 May 2000

Display high resolution version

When high-energy electrons and positrons annihilate each other in the Large Electron Positron (LEP) collider at the CERN laboratory, Geneva, their energy goes to create new particles. Initially, only a few particles are formed, but if these are quarks and antiquarks they do not emerge as individuals. Instead, they "clothe" themselves (through the strong force) with additional quarks and antiquarks to make composite particles - hadrons - which can be detected. Often the initial quark or antiquark gives rise to a spray, or "jet", of many particles, as happens in this computer reconstruction of an annihilation in the DELPHI detector at LEP, at a total energy of 204 GeV.

Two jets from a quark and an antiquark shoot out from the annihilation at the centre of the detector, one upwards and one to the left. However, another quark and antiquark have formed a single particle, called the upsilon, which consists of a bottom quark bound with its antiquark. The upsilon does not live for long before it decays into a muon and an antimuon, which leave the two tracks pointing to the right. The total energy of the muon pair corresponds to the mass of the upsilon particle, while the two jets appear to have come from the decay of a Z particle, carrier of the weak force.

Credit: CERN / DELPHI