Experiments At High­Energy $e+ e-$ Colliders

Linear e+ e- colliders are the most promising candidates for the next generation of lepton colliders. They will allow physics investigations complementary to the LHC, with a centre-of-mass energy ranging from the Z0 mass to 5 TeV, depending on the project. A short overview over the main studies - TESLA, JLC, NLC and CLIC - is given, emphasising the reasoning that has driven their choices. The strong beam­beam interaction leading to a pinch effect and to beamstrahlung is briefly mentioned, together with its effect on luminosity and luminosity spectrum. The possible tradeoff between the latter two is illustrated. At high centre­of­mass energies, also coherent pair creation influences the beam­beam interaction and requires careful detector design to avoid severe background. Also at low energies incoherent pair creation is a significant background source, its effects on the designs of the vertex detector and the masking system are described. Very briefly additional background sources due to two photon production of hadrons and due to neutrons are mentioned.