Evidence for Production of Single Top Quarks and First Direct Measurement of|Vtb|

The D0 Collaboration presents first evidence for the production of single top quarks at the Fermilab Tevatron $p\overline{p}$ collider. Using a $0.9{\mathrm{fb}}^{-1}$ dataset, we apply a multivariate analysis to separate signal from background and measure $\sigma (p\overline{p}\to tb+X,tqb+X)=4.9\pm 1.4\mathrm{pb}$. The probability to measure a cross section at this value or higher in the absence of a signal is $0.035\%$, corresponding to a 3.4 standard deviation significance. We use the cross section measurement to directly determine the Cabibbo-Kobayashi-Maskawa matrix element that describes the $Wtb$ coupling and find $0.68<|V_{tb}|\le 1$ at $95\%$ C.L. within the standard model.