Observation of Single Top Quark Production and Measurement of |Vtb| with CDF

TL;DR

This work reports the first strong observation of electroweak single top quark production in $par p$ collisions at $\, oot 0 o 1.96$ TeV using 3.2 fb$^{-1}$ of data with the CDF II detector. It combines four W+jets-based multivariate analyses (likelihood, matrix elements, neural network, boosted decision trees) with an orthogonal MET+jets channel to maximize sensitivity, yielding a total cross section $\sigma_{s+t}=2.3^{+0.6}_{-0.5}$ pb and a direct extraction of $|V_{tb}|=0.91^{+0.11}_{-0.11}{}^{+-0.07}_{ ext{theory}}$, along with a 95% CL lower limit $|V_{tb}|>0.71$. A two-dimensional fit separates $\sigma_s$ and $\sigma_t$, revealing $\sigma_s=1.8^{+0.7}_{-0.5}$ pb and $\sigma_t=0.8^{+0.4}_{-0.4}$ pb, with indications of tension relative to some SM predictions. The analysis demonstrates the power of advanced multivariate techniques and in-situ background constraints for extracting rare electroweak processes from challenging hadronic environments, informing CKM physics and guiding Higgs searches at hadron colliders.

Abstract

We report the observation of electroweak single top quark production in 3.2 fb-1 of ppbar collision data collected by the Collider Detector at Fermilab at sqrt{s}=1.96 TeV. Candidate events in the W+jets topology with a leptonically decaying W boson are classified as signal-like by four parallel analyses based on likelihood functions, matrix elements, neural networks, and boosted decision trees. These results are combined using a super discriminant analysis based on genetically evolved neural networks in order to improve the sensitivity. This combined result is further combined with that of a search for a single top quark signal in an orthogonal sample of events with missing transverse energy plus jets and no charged lepton. We observe a signal consistent with the standard model prediction but inconsistent with the background-only model by 5.0 standard deviations, with a median expected sensitivity in excess of 5.9 standard deviations. We measure a production cross section of 2.3+0.6-0.5(stat+sys) pb, extract the CKM matrix element value |Vtb|=0.91+0.11-0.11 (stat+sys)+-0.07(theory), and set a lower limit |Vtb|>0.71 at the 95% confidence level, assuming m_t=175 GeVc^2.

Figures (36)

• Figure 1: Representative Feynman diagrams of single top quark production. Figures (a) and (b) are $s$- and $t$-channel processes, respectively, while figure (c) is associated $Wt$ production, which contributes a small amount to the expected cross section at the Tevatron.
• Figure 2: Cutaway isometric view of the CDF II detector.
• Figure 3: Feynman diagrams showing the final states of the dominant $s$-channel (a) and $t$-channel (b) processes, with leptonic $W$ boson decays. Both final states contain a charged lepton, a neutrino, and two jets, at least one of which originates from a $b$ quark.
• Figure 4: Distributions in ($\phi-\eta$) space of the electron (a) and muon (b) selection categories, showing the coverage of the detector that each lepton category provides. The muon categories are more complicated due to the geometrical limitations of the several different muon detectors of CDF.
• Figure 5: Plots of ${{ 0 \dimen@ii00\hbox{$\m@th/$\dimen@ii} \hbox{$\m@th E$}}}_{\rm T,sig}$ vs. $M_{\rm T}^W$ for $W+$jets Monte Carlo, the selected data in the $\ell+ {{ 0 \dimen@ii00\hbox{$\m@th/$\dimen@ii} \hbox{$\m@th E$}}}_{\rm T}$+2 jets sample, and the two distributions subtracted for all CEM candidates. The black lines indicate the requirements which are applied. Events with lower ${{ 0 \dimen@ii00\hbox{$\m@th/$\dimen@ii} \hbox{$\m@th E$}}}_{\rm T,sig}$ or $M_{\rm T}^W$ are not selected.