Prospects for a search of invisible particles produced in association with single-top quarks using a multivariate analysis in proton-proton collisions at $\sqrt{s} = 14$ TeV with the ATLAS detector at the HL-LHC

The expected sensitivity of a search for events with one top quark and missing transverse momentum at the final state is estimated in proton–proton collisions at a centre-of-mass energy $\sqrt{s} = 14$ TeV with the ATLAS experiment at the HL-LHC. A non-resonant production of an invisible dark matter particle in association with a right-handed top quark is considered, where the $W$ boson from the top quark is required to decay into an electron or a muon and a neutrino. An estimated upper limit of 3.4 TeV based on a BDT-discriminant analysis, at 95% CL, assuming an integrated luminosity of 3.0 ab$^{−1}$, is set on the non-resonant monotop cross-section.