A search for electroweak tt^-Wj 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 tt^-Wj_EW process. This process has embedded within it a tW-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 tt^-Wj_EW production process from its strong tt^-W production counterpart. A measured (expected) 95% CL upper limit on the cross section is set at σ_tt-WjEW < 251~fb (230 fb), to be compared with the expected SM cross section of 47.7 fb. Limits are set on the SM Effective Field Theory (EFT) operators O_Ht and O⁽¹⁾_HQ, 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, multi-process and multi-operator EFT contributions.