  The production cross-section of a top quark in association with a $W$ boson
is measured using proton-proton collisions at $\sqrt{s} = 8$ TeV. The dataset
corresponds to an integrated luminosity of 20.2 fb$^{-1}$, and was collected in
2012 by the ATLAS detector at the Large Hadron Collider at CERN. The analysis
is performed in the single-lepton channel. Events are selected by requiring one
isolated lepton (electron or muon) and at least three jets. A neural network is
trained to separate the $tW$ signal from the dominant $t\bar{t}$ background.
The cross-section is extracted from a binned profile maximum-likelihood fit to
a two-dimensional discriminant built from the neural-network output and the
invariant mass of the hadronically decaying $W$ boson. The measured
cross-section is $\sigma_{tW} = 26 \pm 7$ pb, in good agreement with the
Standard Model expectation.
