Observation of Single Top Quark Production and Measurement of |Vtb| with CDF (1004.1181v4)

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.

Overview of "Observation of Single Top Quark Production and Measurement of |V_tb| with CDF"

This paper by the CDF Collaboration presents the observation of single top quark production using 3.2 fb⁻¹ of proton-antiproton collision data at a center-of-mass energy of √s = 1.96 TeV, collected by the Collider Detector at Fermilab (CDF). The research not only confirms the production of single top quarks but also provides a precise measurement of the associated cross section and CKM matrix element |V_tb|.

Experimental Design

The analysis focuses on events characterized by the presence of a final state from a single top quark decay. These events include leptons, missing transverse energy (from neutrinos), and jets, specifically b-jets. The single top quark production channels analyzed are the s-channel and t-channel processes. Distinct discriminant techniques are applied to isolate the signal from significant Standard Model background processes such as W + jets and top-antitop (t\bar{t}) production.

Methodology

The team employed four multivariate analysis techniques to classify events based on their likelihood of being signal-like or background-like:

1. Likelihood Functions (LF): Employs probability density functions for separating signal from background within selected bins of input variables.
2. Matrix Element Method (ME): Utilizes complex subroutines based on parton-level differential cross sections to estimate the likelihood of events emanating from specific signal or background processes.
3. Neural Networks (NN): Implements machine learning models trained using a mix of signal and background samples to create discriminant functions optimized for signal isolation.
4. Boosted Decision Trees (BDT): Leverages an ensemble of decision trees to enhance classification stability and discrimination power through adaptive boosting techniques.

These methods exploit the kinematic features of single top quark events and precisely measure quantities such as invariant masses, scalar sums of energies, angular distributions, and jet flavor qualities.

Results

The analysis yields a measurement of the single top quark production cross section σs+t = 2.3^{+0.6}{-0.5} pb, consistent with the Standard Model predictions. The paper reports a statistically significant observation with a significance of 5.0σ beyond the background-only model.

The extracted CKM matrix element |V_tb| = 0.91^{+0.11}_{-0.11}(stat+sys)±0.07(theory) supports the expectations of the Standard Model. Additionally, the measurement sets a lower limit |V_tb| > 0.71 at 95% confidence, assuming a top quark mass of 175 GeV/c².

Implications

This result completes a comprehensive view of single top quark production in the context of the SM. It improves constraints on the CKM matrix, providing insight into electroweak processes involving the third generation of quarks. The methodology and cross-section measurements can pave the way for further exploration and refinement of top quark physics, including its interplay with potential new physics scenarios beyond the Standard Model.

The successful observation of single top quark production also serves as a benchmark for future investigations into the Higgs boson production processes in the WHlvbb channel, thereby fortifying the search strategies toward major discoveries in particle physics.

Conclusion

The research represents a significant advance in top quark physics, contributing to a precise determination of the production rates and CKM matrix elements pertinent to the top quark. It exemplifies the synergy between theoretical predictions, sophisticated data analysis techniques, and high-energy collider experiments in deepening our understanding of the fundamental structure of matter.