flying in opposite directions. Such an experiment would amount to running two Indianapolis 500 races simultaneously, one with cars circling the track clockwise and the other counterclockwise. In the collisions of particles within such demolition derbies, physicists now hoped to make their finding.

At SLAC, Burton Richter was a particular advocate of this approach. He had built SPEAR, the Stanford Positron-Electron Accelerating Ring, and had used it in finding his psi particle, which contained charm. The nature of SPEAR testified to further advances in physics, for it collided beams of electrons and positrons. The positron, far from being a curiosity as in the days of Carl Anderson, now was a mainstay of physics research. Indeed, in SPEAR the positrons were so numerous that they formed beams and served to discover particles such as the psi, which were newer still.

For higher energies yet, physicists could use antiprotons. Antiprotons, like positrons, are a form of antimatter. These particles have all the properties of their conventional counterparts with one exception: They have opposite electric charges. The positron, for one, has the same mass and spin as the electron; but whereas the electron carries negative charge, that of the positron is positive. Similarly, the antiproton has the same mass and spin as the proton, but the proton carries positive charge, whereas that of the antiproton is negative. Emilio Segre and Owen Chamberlain had discovered the antiproton in 1955, using the Bevatron accelerator at the University of California at Berkeley; it had been one of the prizes of the new era of particle physics. Like the positron, the antiproton too had graduated to become a tool of research. A beam of antiprotons, colliding head on with one of the protons, could offer energies sufficient to create W and Z particles.

The Fermilab accelerator had been using protons since day one; it was a natural candidate for conversion. However, rather than carry through a straightforward modification, the lab's directors proceeded with a far-reaching upgrade that would feature an entirely new ring of magnets. Rather than shooting a 200 GeV beam at a fixed target, the new system would produce beams with energies of 1000 GeV, a trillion electron volts, or TeV. Two such beams, countercirculating, would collide head on to produce a total energy of 2 TeV. Still, this project was not due for completion until 1985, which left European researchers with an opportunity.

The opportunity lay with an existing accelerator in CERN, the Super Proton Synchrotron (SPS). In 1976 it was operating more as a powerful counterpart of the Fermilab system, directing proton beams at fixed



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