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

TL;DR

This paper provides evidence for the production of a W boson and a top quark in proton-proton collisions at 7 TeV, using ATLAS data, with results consistent with the Standard Model predictions.

Contribution

It introduces a novel analysis using boosted decision trees to identify Wt production and measures its cross-section at the LHC.

Findings

Evidence for Wt production at 3.3 sigma significance

Measured cross-section of 16.8 pb with uncertainties

Derived |V_{tb}| consistent with the Standard Model

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.