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

TL;DR

This study searches for dark matter produced alongside a single top quark in proton-proton collisions at 13 TeV using the ATLAS detector, exploring a novel production mode within a two-Higgs-doublet model with a pseudoscalar mediator.

Contribution

First investigation of single top quark associated dark matter production in the 2HDM+$a$ model at the LHC with detailed experimental limits.

Findings

No significant excess observed over Standard Model predictions.

Set 95% CL limits on model parameters.

Analyzed multiple final states with leptons and missing transverse momentum.

Abstract

This 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 $\sqrt{s} = 13$ TeV during LHC Run2 (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.