Abstract
The cross-section for the production of a single top quark in association with a $W$ boson in proton--proton collisions at $\sqrt{s}= 8$ TeV is measured. The dataset corresponds to an integrated luminosity of 20.3 fb$^{-1}$, collected by the ATLAS detector in 2012 at the Large Hadron Collider at CERN. Events containing two leptons and one central $b$-jet are selected. The $Wt$ signal is separated from the backgrounds using boosted decision trees, each of which combines a number of discriminating variables into one classifier. Production of $Wt$ events is observed with a significance of $7.7 \sigma$. The cross-section is extracted in a profile likelihood fit to the classifier output distributions. The $Wt$ cross-section, inclusive of decay modes, is measured to be $23.0 \pm 1.3$(stat.)$^{+3.2}_{-3.5}$(syst.)$\pm 1.1$(lumi.) pb. The measured cross-section is used to extract a value for the CKM matrix element $|V_{tb}|$ of $1.01 \pm 0.10$ and a lower limit of 0.80 at the 95\% confidence level. The cross-section for the production of a top quark and a $W$ boson is also measured in a fiducial acceptance requiring two leptons with $p_{\textrm{T}}>25$ GeV and $|\eta|<2.5$, one jet with $p_{\textrm{T}}>20$ GeV and $|\eta|<2.5$, and $E_{\textrm{T}}^{\textrm{miss}}>20$ GeV, including both $Wt$ and top-quark pair events as signal. The measured value of the fiducial cross-section is $0.85 \pm 0.01$(stat.)$^{+0.06}_{-0.07}$(syst.)$\pm 0.03$(lumi.) pb.
Highlights
The production of single top quarks has been observed at the Tevatron protonantiproton collider in the t-channel [10, 11] and s-channel [12,13,14], as well as their combination [15,16,17]
The binned likelihood function is constructed as the product of Poisson probability terms over all bins considered in the analysis
This likelihood depends on the signal-strength parameter μ, which is a multiplicative factor on the unconstrained W t yield prediction
Summary
The ATLAS detector [37] is a multi-purpose particle detector with a forward-backward symmetric cylindrical geometry and a near 4π coverage in solid angle. ATLAS comprises. The calorimeter transverse energy within a cone of size ∆R = 0.2 and the scalar sum of track pT within ∆R of 0.3 around the electron, in each case excluding the contribution from the electron itself, are each required to be smaller than ET- and η-dependent thresholds calibrated to give nominal selection efficiencies of 90% for prompt electrons from Z → ee decays. The missing transverse momentum (ETmiss) is calculated as the magnitude of the vector sum over the energies of all clusters in the calorimeters, and is refined by applying objectlevel corrections to the contributions arising from identified electrons, muons, and jets [49]
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