Combination of searches for singly produced vectorlike top quarks in $ pp$ collisions at $ \sqrt{s}=13\text{}\text{}\mathrm{TeV}$ with the ATLAS detector

A combination of searches for the single production of vectorlike top quarks ($ T$) is presented. These analyses are based on proton-proton collisions at $ \sqrt{s}=13\text{}\text{}\mathrm{TeV}$ recorded in 2015–2018 with the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of $ 139\text{}\text{}{\mathrm{fb}}^{-1}$. The $ T$ decay modes considered in this combination are into a top quark and either a Standard Model Higgs boson or a $ Z$ boson ($ T\to Ht$ and $ T\to Zt$). The individual searches used in the combination are differentiated by the number of leptons ($ e$, $ \mu $) 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, $ \kappa $, of 0.5, masses below 2.1 TeV are excluded, resulting in the most restrictive limits to date.