Measurement of the cross-section for associated production of a top quark and a W boson at $\sqrt{s}=8$ TeV with the ATLAS detector

A measurement is presented of the cross-section for the associated production of a top quark and a $W$ boson in a proton proton collision ($pp$) data set at a center-of-mass energy $\sqrt{s}=8$ TeV corresponding to an integrated luminosity of 20.3 fb$^{-1}$ with the ATLAS detector at the LHC. Events are selected by requiring one electron and one oppositely-charged muon, significant missing transverse momentum and at least one $b$-tagged central jet. An approach based on Boosted Decision Trees is followed to optimize the discrimination between the single-top quark signal and the dominant background from top quark pairs. A template fit to the final classifier distributions in both 1-jet and 2-jet multiplicity events is then used to measure the signal cross-section. A measured cross-section of $\sigma({Wt})=27.2\pm 5.8$ pb is obtained, corresponding to 4.2 standard deviations evidence with 4.0 standard deviations expected for Standard Model $Wt$ production. From this measurement a lower limit at the 95\% confidence level of 0.72 is derived on the CKM matrix element $|V_{tb}|$, assuming $V_{td}$ and $V_{ts}<< V_{tb}$, but without having to assume three quark generations or CKM matrix unitarity. These results are in good agreement with previous measurements and with the Standard Model expectation.