Abstract: The production of a top quark-antiquark pair in association with a W boson (t¯tW) is measured in proton-proton collisions at a center-of-mass energy of 13TeV. The analyzed data was recorded by the CMS experiment at the CERN LHC and corresponds to an integrated luminosity of 138 fb-1. Events with two or three leptons (electrons and muons) and additional jets are selected. In events with two leptons, a multiclass neural network is used to distinguish between the signal and background processes. Events with three leptons are categorized based on the number of jets and of jets originating from b quark hadronization, and the lepton charges. The inclusive t¯tW production cross section in the full phase space is measured to be 868±40(stat)±51(syst) fb. The t¯tW+ and t¯tW? cross sections are also measured as 553±30(stat)±30(syst) and 343±26(stat)±25(syst) fb, respectively, and the corresponding ratio of the two cross sections is found to be 1.61 ± 0.15(stat) +0.07 ?0.05(syst). The measured cross sections are larger than but consistent with the standard model predictions within two standard deviations, and represent the most precise measurement of these cross sections to date.

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