Combination of searches for singly produced vectorlike top quarks in (Formula presented) collisions at (Formula presented) with the ATLAS detector

A combination of searches for the single production of vectorlike top quarks ((Formula presented)) is presented. These analyses are based on proton-proton collisions at (Formula presented) recorded in 2015-2018 with the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of (Formula presented). The (Formula presented) decay modes considered in this combination are into a top quark and either a Standard Model Higgs boson or a (Formula presented) boson ((Formula presented) and (Formula presented)). The individual searches used in the combination are differentiated by the number of leptons ((Formula presented), (Formula presented)) in the final state. The observed data are found to be in good agreement with the Standard Model background prediction. Interpretations are provided for a range of masses and couplings of the vectorlike top quark for benchmark models and generalized representations in terms of 95% confidence level limits. For a benchmark signal prediction of a vectorlike top quark SU(2) singlet with electroweak coupling, (Formula presented), of 0.5, masses below 2.1 TeV are excluded, resulting in the most restrictive limits to date.