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Two-Fermion Production at LEP2

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.:

ll Feynman diagram.

If a Z/photon decays to quarks, it will usually yield two back-to-back jets of hadrons as in this event. Sometimes an energetic gluon may be emitted, in which case a three-jet event will be seen instead. 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.:

llgamma Feynman diagram

Here is an example of a radiative return to the Z, where the Z decays to quarks (actually the first hadronic event recorded by OPAL at LEP2). The mass of the observed hadronic system is 94 GeV, consistent with the Z boson mass.

At Cambridge, measurement of cross-sections for quark-pairs and electron pairs have been performed at 10 different centre-of-mass energies at LEP2. Here is an example of the cross-sections for quark production, with the interesting non-radiative process shown separately.

Hadronic cross-section

The measurements are in good agreement with the expectations from the Standard Model, and can be used to place constraints on many kinds of new physics beyond the Standard Model.