targets, with energy as great as 400 GeV. During that year, several physicists proposed to convert it to a colliding beam operation by making provision for a countercirculating beam of antiprotons. This conversion was more straightforward than the Fermilab upgrade, but it nevertheless took several years.

Finally, in July 1981 the machine was ready. It produced peak energies of 270 GeV in each beam, or 540 GeV in collisions. Initial operation involved low beam intensities, and these runs failed to produce W or Z particles. That was not surprising; the theory predicted that they would be rare, and the experimenters had only an approximate idea of the energy levels that would produce them. Until they zeroed in on the right energies, these particles would be rarer still.

Carlo Rubbia of Harvard University, who had proposed converting the SPS in 1976, now led the experimenters. They boosted the beam intensity by an order of magnitude, greatly increasing the collision rate, and proceeded with their search. In 1983 they gained full success, detecting the W+ and W as well as the Z. Subsequent worked pinned down their masses: 80.15 GeV for the W particles, 91.187 GeV for the Z. This showed that in an important respect the electroweak theory was stronger than the quark theory. Quark theorists, such as Glashow, had predicted the existence of charm but had not been able to predict the mass of charmed particles. By contrast, electroweak theory had predicted masses as well as existence and with remarkably good accuracy.

Meanwhile, the matter of quark families was coming to the fore. In 1977 Leon Lederman, working at Fermilab, discovered a member of the third family, which received the name of the bottom quark. Its mass proved to be 4.7 GeV, which confirmed a pattern. Quarks of the second family, the charm and strange, had proven to be considerably more massive than their counterparts in the first family. Third-family quarks were heavier still. Indeed, subsequent work showed that the partner of the bottom quark—known quite logically as the top quark—certainly merited its name. Its mass indeed was at the top; searches up to 91 GeV failed to find it. That meant it would be even heavier than the W or Z particles, the heaviest yet found.

There still remained the question of how many such quark families existed in nature. In a virtuoso feat, investigators at SLAC and CERN proceeded to find a direct answer through studies of the Z. In doing so, they introduced a new concept: That studies of specific particles could shed light on fundamental problems that did not involve these particles directly. The multiplicity of families stood as an issue in quark theory; it did not appear within electroweak theory, which represented a separate



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