ATLAS collaboration (2871 authors)Aguilar-Saavedra, Juan AntonioAmor Dos Santos, Susana PatriciaAnjos, NunoAraque, Juan PedroCantrill, RobertCarvalho, JoãoCastro, Nuno FilipeConde Muiño, PatriciaDa Cunha Sargedas De Sousa, Mario JoseFiolhais, MiguelGalhardo, BrunoGomes, AgostinhoGonçalo, RicardoJorge, PedroLopes, LourencoMachado Miguens, JoanaMaio, AméliaManeira, JoséOnofre, AntónioPalma, AlbertoPedro, RutePina, João AntonioSantos, HelenaSaraiva, JoãoSilva, JoséTavares Delgado, AdemarVeloso, FilipeWolters, Helmut2019-02-042019-02-042016-01-11http://dx.doi.org/10.1007/JHEP01(2016)064http://hdl.handle.net/10400.26/26952The cross-section for the production of a single top quark in association with a W boson in proton-proton collisions at $ \sqrt{s}=8\mathrm{T}\mathrm{e}\mathrm{V} $ is measured. The dataset corresponds to an integrated luminosity of 20.3 fb$^{−1}$, collected by the ATLAS detector in 2012 at the Large Hadron Collider at CERN. Events containing two leptons and one central b-jet are selected. The W t signal is separated from the backgrounds using boosted decision trees, each of which combines a number of discriminating variables into one classifier. Production of W t events is observed with a significance of 7.7σ. The cross-section is extracted in a profile likelihood fit to the classifier output distributions. The W t cross-section, inclusive of decay modes, is measured to be 23.0 ± 1.3(stat.)$_{− 3.5}^{+ 3.2}$ (syst.)±1.1(lumi.) pb. The measured cross-section is used to extract a value for the CKM matrix element |V$_{tb}$ | of 1.01 ± 0.10 and a lower limit of 0.80 at the 95% confidence level. The cross-section for the production of a top quark and a W boson is also measured in a fiducial acceptance requiring two leptons with p$_{T}$ > 25 GeV and |η| < 2.5, one jet with p$_{T}$ > 20 GeV and |η| < 2.5, and E$_{T}^{miss}$  > 20 GeV, including both W t and top-quark pair events as signal. The measured value of the fiducial cross-section is 0.85 ± 0.01(stat.)$_{− 0.07}^{+ 0.07}$ (syst.)±0.03(lumi.) pb.engjournal article2019-02-04