Evidence for production of single top quarks and first direct measurement of |Vtb|

Abstract

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

Figures (3)

• Figure 1: Representative Feynman diagrams for (a) s-channel single top quark production and (b) t-channel production.
• Figure 2: Boosted decision tree output distributions for (a) a $W$+jets-dominated control sample, (b) a $t\bar{t}$-dominated control sample, and (c) the high-discriminant region of the sum of all 12 $tb$+$tqb$ DTs. For (a) and (b), $H_T = E_T^{\ell} + {\hbox{$\not\!\! E_T$}} + \sum E_T^{\rm alljets}$. Plot (d) shows the invariant mass of the reconstructed $W$ boson and highest-$p_T$$b$-tagged jet for events with $O_{\rm DT} > 0.65$. The hatched bands show the $\pm 1$ standard deviation uncertainty on the background. The expected signal is shown using the measured cross section.
• Figure 3: Expected SM and measured Bayesian posterior probability densities for the $tb$+$tqb$ cross section. The shaded regions indicate one standard deviation above and below the peak positions.