The observation of single top quark production at the Tevatron was a significant achievement for the CDF and D0 experiments. With these observations, the cross section for electroweak-produced top quarks at a hadron collider was measured for the first time, and due to the direct coupling of the b quark with the singly-produced top quark, an upper limit on the Cabibbo-Kobayashi-Maskawa (CKM) matrix element magnitude V_tb could be placed. The standard model (SM) prediction for the combined s- and t-channel single top quark production cross section has been calculated to next-to-next-to-leading order. Assuming CP conservation, the SM calculation is 3.15 +- 0.36, excluding the contribution from the tW production mode, which is expected to be negligible in this final state. The primary sensitivity to measuring this quantity is usually obtained from events where the W decays leptonically to a lepton-neutrino pair alongside a pair of jets, one of which is b-tagged or identified as likely having originated from a bottom quark. This sample of events provides a distinct signature against backgrounds produced by the strong force (QCD multijet or MJ background), which contain no leptons. A complementary method in measuring the single top quark cross section is through events in the final state that contain two or three jets and significant imbalance in the total transverse momentum (missing transverse energy, MET), which results from the leptonic decay of the W boson, where the lepton has been lost due to reconstruction or acceptance effects. Although MJ events comprise the dominant background in this final state (hereafter METbb analysis or sample), the requirement of significant MET greatly suppresses the MJ background. The first CDF measurement of single top quark production in the METbb final state was performed with a dataset corresponding to an integrated luminosity of 2.1 fb ^-1. A new measurement is presented, using the full CDF data set. All the techniques developed in the search for s-channel single top quark production in the METbb are exploited in this update.

lepton veto = use loose identification cuts to reject events with isolated leptons

MET > 35 GeV

Number of jets = 2 or 3 and one of the leading jets (j1 or j2) central (|&eta| < 0.9).

Events with a larger number of jets are rejected

&Delta R(j1 , j2 ) > 0.8

ET (j1 ) > 25 GeV, ET (j2 ) > 20 GeV

Events are b-tagged either 1T, TT, or TL (T = HOBIT tight (HOBIT value > 0.98) L = HOBIT loose (HOBIT value > 0.72))

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Most physics processes are modeled using Monte Carlo simulation programs. The electroweak single top samples are modeled using the POWHEG generator. Backgrounds from V+jets (where V represents a W or Z boson), W+c, and associated Higgs and W or Z boson (VH) production are modeled using ALPGEN with showering simulated by PYTHIA. Diboson (VV) and strongly produced top-pair events (assuming a top-quark mass of 172.5 GeV) are simulated using PYTHIA. Two remaining backgrounds include contributions from events with falsely-tagged jets (``electroweak mistags'') and MJ events. The electroweak mistag samples are modeled by weighting V+jets and diboson-simulated events by mistag probabilities, derived from dedicated data samples. The dominant background in the METbb sample, however, originates from the MJ background. To model this background, a data-driven method is used: the MJ background is derived by weighting each pretagged data event by a tag-rate probability derived from a MJ-dominated data sample.

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At this stage of the analysis, placing simple requirements on kinematic event properties is not sufficient to separate the electroweak single top signal from the background. A series of multivariate discriminants that take into account nontrivial variable correlations are therefore employed to optimize suppressing the MJ background and to separate the electroweak single top signal from the remaining backgrounds. The same NN_QCD developed for the s-channel search in METbb is reused in this analysis as QCD veto. The s-channel optimized discriminant as used in the METbb s-channel single top search and a new t-channel optimied discriminant are combined to obtain an NN_s+t final discriminant, used to simultaneously separate both s- and t-channel signal processes from the remaining background.

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The modeling of SM backgrounds is tested in several control samples. A first (EWK) control sample is defined containing events with at least one charged lepton that otherwise satisfy the selection criteria. This sample is independent from the signal sample and is sensitive primarily to top-quark pair, V+jets, and, to a lesser extent, VV production. A second (QCD) control sample contains events that do not meet the minimal requirement on the NN_QCD output variable but otherwise satisfy the selection criteria.

two-jet sample

three-jets sample

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Yield Table in 2-jets sample

Yield Table in 3-jets sample