Evidence for production of single top quarks

We present first evidence for the production of single top quarks in the D0 detector at the Fermilab Tevatron $p\overline{p}$ collider. The standard model predicts that the electroweak interaction can produce a top quark together with an antibottom quark or light quark, without the antiparticle top-quark partner that is always produced from strong-coupling processes. Top quarks were first observed in pair production in 1995, and since then, single top-quark production has been searched for in ever larger data sets. In this analysis, we select events from a $0.9\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ data set that have an electron or muon and missing transverse energy from the decay of a $W$ boson from the top-quark decay, and two, three, or four jets, with one or two of the jets identified as originating from a $b$ hadron decay. The selected events are mostly backgrounds such as $W+\mathrm{\text{jets}}$ and $t\overline{t}$ events, which we separate from the expected signals using three multivariate analysis techniques: boosted decision trees, Bayesian neural networks, and matrix-element calculations. A binned likelihood fit of the signal cross section plus background to the data from the combination of the results from the three analysis methods gives a cross section for single top-quark production of $\ensuremath{\sigma}(p\overline{p}\ensuremath{\rightarrow}tb+X,tqb+X)=4.7\ifmmode\pm\else\textpm\fi{}1.3\text{ }\text{ }\mathrm{pb}$. The probability to measure a cross section at this value or higher in the absence of signal is 0.014%, corresponding to a 3.6 standard deviation significance. The measured cross section value is compatible at the 10% level with the standard model prediction for electroweak top-quark production. We use the cross section measurement to directly determine the Cabibbo-Kobayashi-Maskawa quark mixing matrix element that describes the $Wtb$ coupling and find $|{V}_{tb}{f}_{1}^{L}|={1.31}_{\ensuremath{-}0.21}^{+0.25}$, where ${f}_{1}^{L}$ is a generic vector coupling. This model-independent measurement translates into $0.68<|{V}_{tb}|\ensuremath{\le}1$ at the 95% C.L. in the standard model.