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Evidence for the associated production of a W boson and a top quark in ATLAS at sqrt(s) = 7 TeV

ATLAS Collaboration

TL;DR

This study provides evidence for the associated production of a W boson and a top quark in 7 TeV proton-proton collisions using 2.05 fb^-1 of ATLAS data. The analysis focuses on dileptonic final states and employs boosted decision trees to distinguish Wt from tt̄ and other backgrounds, followed by a three-bin template fit to extract the Wt cross-section. The result, sigma_Wt = 16.8 ± 2.9 (stat) ± 4.9 (syst) pb, yields a 3.3σ observed significance and enables a direct determinations of |V_tb| = 1.03^{+0.16}_{-0.19}, consistent with SM expectations and other CKM measurements. The work validates the Wt channel as a probe of W-t-b couplings at the LHC and demonstrates the efficacy of multivariate techniques in separating rare electroweak processes from dominant tt̄ backgrounds.

Abstract

This letter presents evidence for the associated production of a W boson and a top quark using 2.05 fb^-1 of pp collision data at sqrt(s) = 7 TeV accumulated with the ATLAS detector at the LHC. The analysis is based on the selection of the dileptonic final states with events featuring two isolated leptons, electron or muon, with significant transverse missing momentum and at least one jet. An approach based on boosted decision trees has been developed to improve the discrimination of single top-quark Wt events from background. A template fit to the final classifier distributions is performed to determine the cross-section. The result is incompatible with the background-only hypothesis at the 3.3 sigma level, the expected sensitivity assuming the Standard Model production rate being 3.4 sigma. The corresponding cross-section is determined and found to be sigma_Wt = 16.8 +/- 2.9 (stat) +/- 4.9 (syst) pb, in good agreement with the Standard Model expectation. From this result the CKM matrix element |V_{tb}| = 1.03{+0.16}{-0.19} is derived assuming that the Wt production through |V_{ts}| and |V_{td}| is small.

Evidence for the associated production of a W boson and a top quark in ATLAS at sqrt(s) = 7 TeV

TL;DR

This study provides evidence for the associated production of a W boson and a top quark in 7 TeV proton-proton collisions using 2.05 fb^-1 of ATLAS data. The analysis focuses on dileptonic final states and employs boosted decision trees to distinguish Wt from tt̄ and other backgrounds, followed by a three-bin template fit to extract the Wt cross-section. The result, sigma_Wt = 16.8 ± 2.9 (stat) ± 4.9 (syst) pb, yields a 3.3σ observed significance and enables a direct determinations of |V_tb| = 1.03^{+0.16}_{-0.19}, consistent with SM expectations and other CKM measurements. The work validates the Wt channel as a probe of W-t-b couplings at the LHC and demonstrates the efficacy of multivariate techniques in separating rare electroweak processes from dominant tt̄ backgrounds.

Abstract

This letter presents evidence for the associated production of a W boson and a top quark using 2.05 fb^-1 of pp collision data at sqrt(s) = 7 TeV accumulated with the ATLAS detector at the LHC. The analysis is based on the selection of the dileptonic final states with events featuring two isolated leptons, electron or muon, with significant transverse missing momentum and at least one jet. An approach based on boosted decision trees has been developed to improve the discrimination of single top-quark Wt events from background. A template fit to the final classifier distributions is performed to determine the cross-section. The result is incompatible with the background-only hypothesis at the 3.3 sigma level, the expected sensitivity assuming the Standard Model production rate being 3.4 sigma. The corresponding cross-section is determined and found to be sigma_Wt = 16.8 +/- 2.9 (stat) +/- 4.9 (syst) pb, in good agreement with the Standard Model expectation. From this result the CKM matrix element |V_{tb}| = 1.03{+0.16}{-0.19} is derived assuming that the Wt production through |V_{ts}| and |V_{td}| is small.

Paper Structure

This paper contains 9 sections, 6 equations, 3 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Data and Monte Carlo simulation
  3. Event reconstruction and selection
  4. Background estimation
  5. Discriminating variables for $Wt$ events
  6. Cross-section determination
  7. Determination of $|V_{\it{tb}}|$
  8. Conclusion
  9. Acknowledgements

Figures (3)

  • Figure 1: Leading-order Feynman diagrams for associated production of a single top-quark and a $W$ boson.
  • Figure 2: \ref{['FIG-BDT-INPUT-a']} Number of jets with $p_{\rm T}>30~$Ge V and $|\eta|<2.5$ after the selection; hatched bands show the jet energy scale (JES) uncertainty. The $Wt$ signal is normalised to the theory prediction. \ref{['FIG-BDT-INPUT-b']} Distribution of BDT output for the signal ($Wt$-channel) and background ($t\bar{t}$ diboson, Drell-Yan and fake dileptons) in signal enriched 1-jet bin. The BDT method uses 2 statistically independent sets of MC-simulated events, indicated as training and testing samples, to check both signal and background BDT output stability. The BDT weight file is derived from a training sample and applied to a testing sample.
  • Figure 3: BDT output for selected events in \ref{['FIG-BDT-FINAL-a']} 1-jet, \ref{['FIG-BDT-FINAL-b']} 2-jet and \ref{['FIG-BDT-FINAL-c']}$\ge$3-jet categories. The $Wt$ signal is normalised to the theory prediction in all three categories.