1991 – LEP experiments show that there are only three light neutrinos

When the LEP collider started operation in 1989 it accelerated the electrons and positrons to a total energy of 45 GeV each to enable production of the Z Boson, which has a mass of approximately 91 GeV. The accelerator was upgraded later to enable production of a pair of W Bosons, each weighing approximately 80 GeV. LEP collider energy eventually topped at 104 GeV at the end in 2000. At the end of 2000, LEP was shut down and then dismantled in order to make room in the tunnel for the construction of the Large Hadron Collider (LHC).

The Super Proton Synchrotron (an older ring collider) is used to accelerate electrons and positrons to nearly the speed of light. These are then injected into the ring. As in all ring colliders, the LEP’s ring consists of many magnets which force the charged particles into a circular trajectory (so that they stay inside the ring), RF accelerators which accelerate the particles with radio frequency (RF) waves and quadrupoles that focus the particle beam (i.e. keep the particles together). (Note that ‘accelerating’ here does not really mean that the particles get much faster, as they already are very close to the speed of light in the beginning. But they gain a lot of kinetic energy and so become more massive because mass and energy are equal according to the theory of special relativity.) When the particles are accelerated to maximum energy (and focused to so-called bunches), an electron and a positron bunch is made to collide with each other at one of the collision points of the detector. When an electron and a positron collide, they annihilate to a virtual particle, either a photon or a Z boson. The virtual particle almost immediately decays into other elementary particles, which are then detected by huge particle detectors.

The results of the LEP exeriments allowed precise values of many quantities of the Standard Model — most importantly the mass of the Z boson and the W boson (which were discovered in 1983 at an earlier CERN collider) to be obtained — and so confirm the Model and put it on a solid basis of empirical data. Precision measurements of the shape of the Z boson mass peak constrained the number of light neutrinos in the standard model to exactly three.