Aad, G.Abbott, B.Abbott, D. C.Abud, A. AbedAbeling, K.Abhayasinghe, D. K.Çetin, Serkant Ali2024-07-182024-07-1820211434-60441434-6052https://doi.org/10.1140/epjc/s10052-021-09566-yhttps://hdl.handle.net/11411/7914This paper presents a search for dark matter in the context of a two-Higgs-doublet model together with an additional pseudoscalar mediator, a, which decays into the dark-matter particles. Processes where the pseudoscalar mediator is produced in association with a single top quark in the 2HDM+a model are explored for the first time at the LHC. Several final states which include either one or two charged leptons (electrons or muons) and a significant amount of missing transverse momentum are considered. The analysis is based on proton-proton collision data collected with the ATLAS experiment at root s = 13 TeV during LHC Run 2 (2015-2018), corresponding to an integrated luminosity of 139 fb(-1). No significant excess above the Standard Model predictions is found. The results are expressed as 95% confidence-level limits on the parameters of the signal models considered.eninfo:eu-repo/semantics/openAccessMeasuring MassesPlus PlusArticle2-s2.0-8511650195310.1140/epjc/s10052-021-09566-y10Q181Q2WOS:000702719500002