Search for dark matter associated production with a single top quark in $\sqrt{s}=13$ TeV (pp) collisions with the ATLAS detector

This paper presents a search for dark matter in the context of a two-Higgs-doublet model together with an additional pseudo-scalar mediator, which decays to the dark matter particles ($\chi$). Processes where the pseudo-scalar mediator is produced in association with a single top quark are explored for the first time at the LHC. Several final states are considered, including either one or two charged leptons (electrons or muons) and a significant amount of missing transverse momentum. The analysis is based on the data collected with the ATLAS experiment at $\sqrt{s} = 13$ TeV during LHC Run~2 ($2015-2018$), corresponding to an integrated luminosity of 139 fb$^{-1}$. No significant excess is found with respect to Standard Model predictions. The results are expressed as 95% confidence level limits on the parameters of the signal models considered.