Probing dark matter couplings to top and bottom quarks at the LHC

Monojet searches are a powerful way to place model-independent constraints on effective operators coupling dark matter to the standard model. For operators generated by the exchange of a scalar mediator, however, couplings to light quarks are suppressed and the prospect of probing such interactions through the inclusive monojet channel at the LHC is limited. We propose dedicated searches, focusing on bottom and top quark final states, to constrain this class of operators. We show that a search in mono b-jets can significantly improve current limits. The mono-b signal arises partly from direct production of b quarks in association with dark matter, but the dominant component is from top quark pair production in the kinematic regime where one top is boosted. A search for tops plus missing energy can strengthen the bounds even more; in this case signal and background have very different missing energy distributions. We find an overall improvement by several orders of magnitude in the bound on the direct detection cross section for scalar or pseudoscalar couplings.