Search for monotop events using the ATLAS detector at the LHC

A search for the production of single top quarks in association with missing energy is performed in proton-proton collisions at a centre-of-mass energy of $\sqrt{s}=\mathrm{8~TeV}$ with the ATLAS experiment at the Large Hadron Collider using data collected in 2012, corresponding to an integrated luminosity of $20.3$ fb$^{-1}$. In this search, the $W$ boson from the top quark is required to decay into an electron or a muon and a neutrino. No deviation from the Standard Model prediction is observed, and upper limits are set on the production cross-section for resonant and non-resonant production of an invisible exotic state in association with a right-handed top quark. In the case of resonant production, for a spin-$0$ resonance with a mass of $500$ GeV, an effective coupling value above $0.15$ is excluded at 95$\%$ CL for the top quark and an invisible spin-$1/2$ state with mass between $0$ GeV and $100$ GeV. In the case of non-resonant production, an effective coupling value above $0.2$ is excluded at 95$\%$ CL for the top quark and an invisible spin-$1$ state with mass between $0$ GeV and $657$ GeV.