Search for electroweak t t ¯ W j production in multileptonic final states at s = 13 TeV with the ATLAS detector and bounds on effective field theory operators

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 t t ¯ W j EW process. This process has embedded within it a t W -scattering 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 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 t t ¯ W j EW production process from its strong t t ¯ W production counterpart. A measured (expected) 95% CL upper limit on the cross section is set at σ t t ¯ W j EW < 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 H t and O H Q ( 1 ) , which modify the electroweak couplings of the top quark through contributions to the t W -scattering vertex. The interpretation acts as a case study to emphasize the importance of energy-dependent sensitivity, multiprocess, and multioperator EFT contributions.