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DELPHI is one of four large particle detectors at the Large Electron Positron Collider (LEP) at CERN. The operation of the LEP collider at CERN started in August 1989 opening a new chapter in the history of particle physics. With a circumference of 27 km, LEP is the largest accelerator yet built. LEP was closed in November 2000 to make room for the Large Hadron Collider, expected to be ready in 2006. LEP accelerates electrons and their anti-particles, positrons, in opposite directions in a vacuum pipe inside aring of magnets, before inducing them to collide head-on. In their annihilation, the energy released in a small volume is comparable to that which existed in the Universe a fraction of a second after the Big Bang. The initial energy of the LEP collider was 91 GeV in order to produce the Z0 particle, the mediator of the weak interaction. The Z particle and its charged partner the W, both discovered at CERN in 1983, are responsible for the weak interaction, which in addition to being responsible for many of the radioactive decays is responsible for the energy production of the Sun. The theory of fundamental particles, the Standard Model, has been thoroughly tested in the DELPHI experiment by studying the creation and decay of the Z0 particle. The Z0 particle is very short-lived, so its presence has to be inferred from the particles it decays into. In 1995 the energy of the LEP collider was doubled to be able to produce Z0Z0 and of W+W-particle pairs, thus opening a new domain of investigations and testing of the Standard Model. In addition the large collision energy made it possible to continue the search for new particles, like the Higgs particle and supersymmetric particles.
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