Current estimates put Dark Matter to 26% of the energy-matter content of the universe,

but very little is known about it other than its gravitational interactions. Eorts to learn

more about Dark Matter include searching for it at high energy particle colliders. The lack

of information about the nature of Dark Matter makes this a complicated task, and many

searches are performed in dierent channels, and considering dierent theoretical models. In

this thesis, I explore two such analyses, performed in the ATLAS collaboration using data

from the ATLAS detector at the Large Hadron Collider at CERN: the tW+MET (missing

transverse energy) nal state and the tt+MET nal state. I have made a generation-level

study of the overlap between the signal regions used, and come to the conclusion that there

is some. I have also compared the models used in these analyses, the 2HDM+a and the

simplied spin-0 pseudoscalar model. Given the simplications made in my study, however,

more sophisticated approaches should be used before anything conclusive can be said.