Search for dark matter produced in association with a top quark pair at $\sqrt{s}$ = 13 TeV

A search is performed for dark matter produced in association with a top quark pair in proton-proton collisions at $\sqrt{s}$ = 13 TeV recorded by the CMS detector at the LHC in 2016. The data correspond to an integrated luminosity of $35.9~\mathrm{fb}^{-1}$. Combined results of final states involving zero, one, or two leptons (electrons or muons) are presented. No significant excess is observed, and the results are interpreted in the context of simplified models of dark matter production via spin-0 mediators. Scalar (pseudoscalar) mediators with masses below 165 (223) GeV are excluded at $95\%$ confidence level. These are the most stringent limits at the LHC on dark matter models involving a scalar mediator. Constraints on the coupling strength between dark matter and standard model quarks are also presented.