Unpaired top quarks Available to Purchase

Weighing 200 times as much as a proton, the top quark is by far the heaviest elementary particle known. Because the strong nuclear force can't change a quark's flavor, it can produce quarks only in pairs with their antiquarks. The weak force can change flavors. But weak-interaction cross sections are so small that it's almost impossible to produce any quark, let alone the heaviest and rarest, without its antiquark in collisions between hadrons. But the DZero detector collaboration at Fermilab's Tevatron collider seems to have managed it. From among 10¹⁴ high-energy proton– antiproton collisions, the collaboration has found evidence for about 60 collisions that produced an unpaired top quark. One can't actually point to the individual events within the sample of 1400 selected candidates. The experimenters' case is indirect and sophisticated, involving complex decision trees and Bayesian neural networks to deduce the fraction of true single-top events buried within...