Search for dark matter produced in association with a single top quark or a top quark pair

A search for dark matter produced in association with top quarks in proton-proton collisions at a center-of-mass energy of $\sqrt{s}=13~\mathrm{TeV}$ is presented. The dataset used corresponds to an integrated luminosity of $35.9~\mathrm{fb}^{-1}$ recorded with the CMS detector at the LHC. Whereas previous searches for scalar or pseudoscalar interactions considered dark matter production in association with a top quark pair only, this analysis also includes production modes with a single top quark. The results are derived from a statistical combination of the contributions from multiple signal regions that are defined to target either the single top or top quark pair final states. No significant deviations with respect to standard model predictions are observed. The results are interpreted in the context of simplified models where a new scalar or pseudoscalar mediator particle couples to the top quark and subsequently decays into dark matter particles. Masses below 290 GeV and 300 GeV are excluded at $95\%$ confidence level for scalar and pseudoscalar mediator particles respectively, assuming a dark matter mass of 1 GeV and mediator couplings to fermions and dark matter particles equal to unity.