  We show how studies relevant for mono-X searches at the LHC in simplified
models featuring a dark matter candidate and an $s$-channel mediator can be
performed within the MadGraph5_aMC@NLO framework. We focus on gluon-initiated
loop-induced processes, mostly relevant to the case where the mediator couples
preferentially to third generation quarks and in particular to the top quark.
Our implementation allows us to study signatures at hadron colliders involving
missing transverse energy plus jets or plus neutral bosons ($\gamma,Z,H$),
possibly including the effects of extra radiation by multi-parton merging and
matching to the parton shower.
