  We consider the production of dark matter in association with a pair of top
quarks, mediated by a scalar or pseudoscalar particle in a generic Simplified
Model. We demonstrate that the difference of azimuthal angle between the two
leptons $\Delta \phi_{ll}$, in the dileptonic top decay mode, can directly
probe the CP-properties of the mediator. We estimate the constraints to
strength and CP-structure of dark matter production for these well-motivated
Simplified Models from the LHC Run II.
