Abstract
A search is presented for the electroweak production of a top-quark pair in association with a W boson and at least one additional jet known as the tt̄ Wj(EW) process. This process has embedded within it a tWscattering vertex, which is probed directly for the first time. The collision data were collected with the ATLAS detector during Run 2 of the LHC and correspond to an integrated luminosity of 140 fb(-1) at root s = 13 TeV. The search uses same-charge pairs of electrons and muons together with jets, of which at least one is b-tagged. The properties of the most forward jet relative to the rest of the event are used to discriminate the electroweak tt̄ Wj(EW) production process from its strong tt̄W production counterpart. A measured (expected) 95% CL upper limit on the cross section is set at st sigma(tt̄ WjEW) < 251 fb (230 fb), to be compared with the expected Standard Model (SM) cross section of 47.7 fb. Limits are set on the SM effective field theory (EFT) operators O-Ht and O-HQ((1)) , which modify the electroweak couplings of the top quark through contributions to the tW-scattering vertex. The interpretation acts as a case study to emphasize the importance of energy-dependent sensitivity, multiprocess, and multioperator EFT contributions.