The first Z particle

6 May 1998

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Fifteen years ago, on 30 April 1983, the UA1 experiment at CERN, the European Laboratory for Particle Physics, first observed the decay of a Z particle, the uncharged carrier of the weak force between particles. UA1 recorded collisions at CERN between high energy protons and antiprotons, and on rare occasions, the proton and antiproton interacted in just the right way to form a Z. Nearly 100 times heavier than a proton, the Z decays almost immediately to lighter particles, but it leaves a characteristic signature in a detector. This computer reconstruction shows the tracks of charged particles thrown out from the proton-antiproton collision. The two white tracks ( towards top right, and almost directly downwards) reveal the Z's decay. They are the tracks of a high-energy electron and an antielectron (positron). Together they carry the mass that once was the Z particle, but converted now to energy, according to E=mc2. The UA1 experiment was built and run by a team led by Carlo Rubbia, who received the Nobel Prize for the discovery of the Z and the related charged particle, the W.

Credit: CERN Photo

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