New Physics searches at the LHC rely very heavily on the precision and accuracy of Standard Model background predictions. Applying the spin-$0$ $s$-channel mediator model, we assess the importance of properly modelling such backgrounds in $tbar{t}$ associated Dark Matter production and discuss higher-order corrections and off-shell effects for the $tbar{t}$ and $tbar{t}Z$ background processes in the presence of extremely exclusive cuts. Exclusion limits are calculated for state-of-the-art NLO full off-shell $tbar{t}$ and $tbar{t}Z$ predictions and compared to those computed with backgrounds in the NWA and / or at LO. We perform the same comparison for several new-physics sensitive observables and evaluate which of them are affected by the top-quark modelling. Additionally, we make suggestions as to which observables should be used to obtain the most stringent limits assuming integrated luminosities of $300$ fb$^{-1}$ and $3000$ fb$^{-1}$.
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