Precision Electroweak Physics at Electron-Positron Colliders
Electron-Positron Colliders have played a key role in the development of m- ern particle physics. The ?rst particle accelerators designed for the study of + ? e e collisions were the machines ADONE (Frascati), SPEAR (SLAC) and DORIS (DESY). They provided clean events originating from a well-de?ned + ? initial state, a great advantage typical to all following e e colliders. Mo- over, in contrary to hadron colliders, here the scattering of point-like particles is studied, which can be calculated with very high precision in theory. There- + ? fore the e e colliders are ideal places for accurate measurements, especially for precision tests of the electroweak Standard Model. On the other hand, the discovery of charmonium and the tau lepton at SPEAR (SLAC) and of the + ? gluon at PETRA (DESY) showed that in certain circumstances e e col- sions are not limited to precision physics, but can also lead to discoveries. In addition, the PETRA accelerator was the ?rst electron-positron collider that contributed to the test of electroweak interactions. For example, measuring + ? + ? the forward-backward asymmetry in muon-pair production, e e ? µ µ, showedcontributionsfromtheexchangeofthe,atthattimestillundiscovered, Z boson, the gauge boson of the weak neutral current. At the Large Electron-Positron Collider, LEP, and the Stanford Linear + ? Collider, SLC, the ?rst e e colliders reaching centre-of-mass energies of 91 GeV, the resonance production of Z bosons was explored.