  The inclusive cross-section for the associated production of a $W$ boson and
top quark is measured using data from proton-proton collisions at
$\sqrt{s}={13}$ TeV. The dataset corresponds to an integrated luminosity of 3.2
fb$^{-1}$, and was collected in 2015 by the ATLAS detector at the Large Hadron
Collider at CERN. Events are selected requiring two opposite sign isolated
leptons and at least one jet; they are separated into signal and control
regions based on their jet multiplicity and the number of jets that are
identified as containing $b$ hadrons. The $Wt$ signal is then separated from
the $t\bar{t}$ background using boosted decision tree discriminants in two
regions. The cross-section is extracted by fitting templates to the data
distributions, and is measured to be $94 \pm 10$ (stat.)$^{+28}_{-22}$ (syst.)
$\pm 2$ (lumi.) pb. The measurement is in agreement with the Standard Model
prediction of $\sigma_{\mathrm{theory}} = 71.7 \pm 1.8\,(\mathrm{scale})\,\pm
3.4\,(\mathrm{PDF})\,\mathrm{pb}$.

High Energy Physics - Experiment