  A search is performed for dark matter particles produced in association with
a top quark pair in proton-proton collisions at $\sqrt{s} =$ 13 TeV. The data
correspond to an integrated luminosity of 35.9 fb$^{-1}$ recorded by the CMS
detector at the LHC. No significant excess over the standard model expectation
is observed. The results are interpreted using simplified models of dark matter
production via spin-0 mediators that couple to dark matter particles and to
standard model quarks, providing constraints on the coupling strength between
the mediator and the quarks. These are the most stringent collider limits to
date for scalar mediators, and the most stringent for pseudoscalar mediators at
low masses.
