Prospects for a search of invisible particles produced in association with single-top quarks with the ATLAS detector at the HL-LHC

The expected sensitivity of a search for events with one top quark and large missing transverse momentum is estimated using simulated proton–proton collisions at a centre-of-mass energy of 14 TeV with the ATLAS experiment at the HL-LHC. A non-resonant production of an exotic state decaying to a pair of invisible dark-matter particles in association with a right-handed top quark is considered. Only the topologies where the $W$ boson from the top quark decays into an electron or a muon and a neutrino are considered. Assuming an integrated luminosity of 3000 fb$^{-1}$, the expected exclusion limit (discovery reach) at 95% CL on the mass of the exotic state is 4.6 TeV (4.0 TeV) using a multivariate analysis based on a boosted decision tree.