|Measurement of the Single Top Quark Production Cross Section in the Missing Transverse Energy plus Jets Sample with the Full CDF II Data Set|
Giorgio Bellettini, Daniela Bortoletto, Matteo Cremonesi, Tom Junk, Kyle Knoepfel, Qiuguang Liu, Fabrizio Margaroli, Karolos Potamianos, Marco Trovato [Contact]
CDF Public Note
A measurement of the single top quark production cross section is present using the full CDF data set corresponding to 9.5 fb -1 of integrated luminosity using a data sample composed of events with an imbalance in the total transverse momentum, b-tagged jets, and no identified leptons. This measurement is an update of a previous result that used one quarter of the complete CDF data set. With respect to the previous analysis, this analysis uses the same strategy applied in the search for s-channel single top quark production in the same topology. A combined s- and t-channel single top quark cross section of 3.53 +1.25 -1.16 (stat+syst) pb is measured and the Cabibbo-Kobayashi-Maskawa matrix element Vtb with a 95% credibility level lower limit at Vtb > 0.63 is also extracted. The total uncertainty of this measurement is 40% less than that of the previous measurement. The t-channel single top quark production cross section, considering the s-channel production as background constrained to the SM prediction, is measured to be 1.19 +0.93 -0.97 (stat+syst) pb.
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 Vtb 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))
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.
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 NNQCD 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 NNs+t final discriminant, used to simultaneously separate both s- and t-channel signal processes from the remaining background.
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 NNQCD output variable but otherwise satisfy the selection criteria.
To measure the signal contribution, the sum of modeled contributions is fitted to the observed data as a function of the final discriminant variable accounting for statistical and systematic uncertainties. The dominant systematic uncertainties arise from the normalization of the V-plus-heavy-flavor background contributions (30%), differences in b-tagging efficiencies between data and simulation (8--16%), and mistag rates (20--30%). Other uncertainties are on the top-antitop pair (3.5%), VV (6%), VH (5%), and W+c (23%) cross sections, normalizations of the QCD multijet background (3--7%), luminosity measurement (6%)~, jet-energy scale (1--6%), trigger efficiency (1--3%), parton distribution functions (2%), and lepton vetoes (2%). The shapes obtained by varying the tag-rate probabilities by one standard deviation from their central values are applied as uncertainties on the shapes of the NNs+t output distribution for the MJ background. Changes in the shape of the NNs+t distribution originating from jet energy scale uncertainties are also incorporated for processes modeled via the simulation.
The result of the binned maximum likelihood fit is shown below. All sources of systematic uncertainties (normalization and shape) are included. The Vtb magnitude is also extracted from the posterior probability density.
|σs+t = 3.53 +1.25 -1.16 pb|
|σt = 1.19 +0.93 -0.97 pb|