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LEP2 Physics

At LEP1, the collision energy was set equal to the mass of the Z-boson, 91 GeV, so most of the events observed involved the formation of the Z. In November 1995 the LEP energy was increased to 130-140 GeV, above the Z mass, but below the energy needed to create a W⁺W^- pair.

In LEP2, the collision energy was increased to 161 GeV (July 1996) and beyond. The main purpose is to have sufficient energy to create pairs of W bosons, the charged gauge bosons of the electroweak theory. These can be produced by the following mechanisms:

The WWZ and WWphoton vertices in these diagrams are precisely predicted by the Standard Model, so observations of W⁺W^- production can be used to test the theory. The mass of the W boson is also precisely predicted using the Standard Model and LEP1 data, so a measurement of the W mass will be a powerful check of the theory. Each W boson decays either to lepton and neutrino, or to quark and antiquark. The signatures of W⁺W^- production are therefore four jets of hadrons, two jets of hadrons together with an energetic isolated lepton and missing energy, or a pair of leptons with missing energy.

Many of the events recorded are similar to those at LEP1, where a lepton pair or quark-antiquark pair is produced in the decay of a virtual Z boson or photon, e.g.:

In addition, many of the events show "radiative return"; the scattering is especially favoured if one or more photons are emitted from the initial e⁺ or e^-, so that the energy is reduced to the mass of the Z boson, e.g.:

Other processes, which were comparatively rare at LEP1, become more common at LEP2. For example, the production of a pair of photons:

which provides a stringent test of QED, and could give indication of composite structure of the electron.