In 1995 Fermilab first produced top quarks, the heaviest and most elusive of the six quark types, in collisions between protons and antiprotons that generated both the top quark and its antimatter twin. These top-antitop pairs form via the so-called strong force, which binds quarks together. Very rarely, according to the Standard Model of particle physics, top quarks may emerge in collisions via the weak force, which causes radioactive decay and can convert one flavor of quark to another. Such weakly made tops, however, would come without their antitop companions (instead a different antiquark, an antibottom quark, forms with the top quark).
The world's biggest accelerator, the Large Hadron Collider (LHC) at CERN, the European laboratory for particle physics near Geneva, will come on line in a few months. Even so, for the next few years it may have a hard time upstaging the Tevatron collider at Fermi National Accelerator Laboratory (Fermilab) in Batavia, Ill., which appears to have generated "single" top quarks. The finding, reported last December, helps to narrow the search for the long sought after Higgs particle and raises the possibility that Fermilab will find it before the LHC does.