The Elusive Top Quark

For years, the validity of the Standard Model was open to question because the reality of quarks could not be verified. Quarks produced in particle accelerators leave only indirect evidence of their vanishingly brief existence, so it took nearly a decade of experimentation before most physicists accepted quarks as real particles. But by the 1980's, the quark theory was firmly established, and researchers had found convincing evidence for the up, down, charmed, strange, and bottom quarks. The top quark, however, continued to elude them.

What made the top quark so hard to find was its enormous mass, now estimated at 176 GeV. (Because of the equivalence of mass and energy, the mass of a particle is usually expressed as the number of electronvolts needed to create it.) Since the top quark could be produced only in a pair with an antitop quark, more than 350 GeV of energy was required for its creation. Until 1985, no accelerator in the world, including the one at Fermilab, was powerful enough to produce such energies. In that year, Fermilab completed modifications to its accelerator ring that changed it from a single-beam accelerator to one that uses two beams of particles—one of protons, the other of antiprotons. The beams are accelerated in opposite directions to just a shade under the speed of light (about 300,000 kilometers [186,000 miles] a second) and then brought to a sharp focus. There, many of the protons and antiprotons collide and disappear in violent flashes of energy.