The subatomic elementary particles known as quarks are among the building blocks of the standard model (SM) of particle physics. Protons and neutrons are made of the lightest, up and down quarks, but the unstable strange, charm, bottom, and top quarks can also be produced in high-energy particle accelerators. The enigmatic top quark-which is the heaviest elementary particle, weighing as much as a gold nucleus-has only been studied at Fermilab's Tevatron collider near Chicago and at the Large

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... ron Collider at CERN, Geneva. It is most often produced with its antiparticle (antitop) through the strong interaction, but it also appears singly through the electroweak interaction, as was demonstrated in 2009 [1, 2] . Two new papers in Physical Review Letters [3, 4] report the first observations of single top quarks coming out of less common electroweak reactions, or "channels." The results fall within SM predictions, but further studies of the relative frequency of these rare channels may reveal new physics, such as a hidden generation of quarks or unexpected top-quark couplings. Although it fits into the SM as the partner of the bottom quark, the top quark appears anomalous because of its extremely large mass of 173 giga-electron-volts (GeV), about 40 times that of the bottom quark. Its mass is also close to those of the W and Z bosons, the force carriers of the electroweak interaction, and that of the recently discovered Higgs boson, leading to speculation that the top quark may play some role in triggering electroweak symmetry breaking, which confers mass on otherwise massless particles. One of the ways to study the top quark is to look at how it appears in particle collision debris. Theory predicts that electroweak interactions will produce top quarks in three distinct channels (see Fig. 1 ). The most common channel-and the only one previously isolated-is the t channel, in which a bottom quark transforms into a top quark through the exchange of a W boson with another quark. In the rarer s channel, a quark and antiquark convert-through an intermediate W boson-into a top and an antibottom. The last single-top process is the tW channel, given this name because it produces a top and W boson from the interaction of a bottom quark and gluon. The rate of occurrence for these different channels is governed by elements of the Cabibbo-Kobayashi-Maskawa (CKM) matrix, which connects the electroweak interactions of all the quarks [5, 6] . In the standard model, this is a 3 × 3 matrix, with one row and column for each of the three doublets, or generations, of quarks. As can be seen in Fig. 1 , all of the single-top channels include an interaction between a top quark, a bottom quark, and a W boson. The strength of this W tb interaction, or vertex, is given by the CKM matrix element V tb . Current estimates-based on a global fit of all the available data within the SM framework-suggest V tb > 0.99. However, this assumes that the CKM matrix is complete; i.e., there are no other quarks. A fourth, asyet-undiscovered generation of quarks would enlarge the CKM matrix and likely affect the value of V tb . Observations of single top quarks can provide direct measurements of V tb and thereby test for additional structure in the CKM matrix, or give evidence for other unexpected processes involving top quarks [7] . Single-top production is a very rare process, happening at the Tevatron about once in every 10 10 proton-