14-13-3: LEP’s winning combination. Thirty years ago, on 14 July to be precise, the Large Electron-Positron collider (LEP) was commissioned. Less than a month later on 13 August, the world’s largest accelerator recorded its first collisions. Then, at a seminar on 13 October, its four experiments announced a result that was fundamental in more ways than one: their measurements of the Z boson, the messenger particle of the weak force, showed that three (and only three) light neutrinos exist. This was a measurement with huge implications for our understanding of the infinitesimally small and the universe itself. In the space of just three weeks, LEP had already produced some 10 000 Z bosons and a spectacular result.
Commissioned at the end of a great human and scientific endeavour, LEP would go on to produce some 18 million Z bosons between 1989 and 1996, testing the electroweak interactions with exceptional precision. The collider also provided many results on quantum chromodynamics, the theory that describes the strong force. With an upgraded acceleration system, LEP saw its collision energy increase considerably, allowing it to produce pairs of W+ and W− bosons, the two other messenger particles of the weak force, from 1996 onwards. Its measurements also made it possible to predict the mass of the top quark, discovered in 1995, and gave hints about the mass of the Higgs boson, discovered in 2012.
The world’s most powerful electron-positron collider was decommissioned in 2000 to make way for the LHC accelerator. LEP tested the Standard Model with unprecedented precision and is still held up as an example for future colliders.
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