Measurement of single top-quark production in association with a $W$ boson in $pp$ collisions at $\sqrt{s}=13$ 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 fb${}^{-1}$ of proton-proton collision data collected with the ATLAS detector at $\sqrt{s}=13$ 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 $t\bar{t}$ background. The cross-section is extracted using a profile likelihood fit to the signal and control regions and it is measured to be ${\sigma }_{tW}=75_{-14}^{+15}$ 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 CKM matrix element $\mathrm{\mid }{f}_{\text{LV}}{V}_{tb}\mathrm{\mid }$ of $0.97\pm 0.10$.