Measurement of single top-quark production in association with a $W$ boson in $pp$ collisions at $\sqrt{s}=13\text{}\text{}\mathrm{TeV}$ with the ATLAS detector

The inclusive cross section for the production of a single top quark in association with a $W$ boson is measured using $140\text{}\text{}{\mathrm{fb}}^{-1}$ of proton-proton collision data collected with the ATLAS detector at $\sqrt{s}=13\text{}\text{}\mathrm{TeV}$. Events containing two charged leptons and at least one jet identified as originating from a $b$-quark are selected. A multivariate discriminant is constructed to separate the $tW$ signal from the $t\overline{t}$ background. The cross section is extracted using a profile likelihood fit to the signal and control regions and it is measured to be ${\sigma }_{tW}=7{5}_{-14}^{+15}\text{}\text{}\mathrm{pb}$, in good agreement with the Standard Model prediction. The measured cross section is used to extract a value for the left-handed form factor at the $Wtb$ vertex times the Cabibbo-Kobayashi-Maskawa matrix element $|{f}_{\mathrm{LV}}{V}_{tb}|$ of $0.97±0.10$.