The tunnel of the Super Proton Synchrotron (SPS) accelerator at CERN, the European particle physics laboratory outside Geneva. The SPS can accelerate protons up to an energy of 400 GeV & then fire them at a fixed target; or, as in more advanced experiments, it can accelerate bunches of protons & antiprotons in opposite directions within the same beam pipe & then cause them to collide with energies of up to 400 GeV per bunch. Operating in this "collider" mode, the SPS made possible the discoveries of the W & Z particles in 1983 and of the top quark in 1984. The SPS forms a ring 7 km in circumference beneath Swiss & French farmland.
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