Search for the Production of Dark Matter in Association with Top Quark Pairs in the Single-lepton Final State in pp collisions at $\sqrt{s} = 8$ TeV

A search for dark matter produced in association with a pair of top quarks with the CMS detector at the LHC, using 19.7 fb$^{-1}$ of pp collision data recorded at $\sqrt{s} = 8$ TeV, is presented. This search requires the presence of one lepton, multiple jets and large missing transverse energy. No excess of events was found above the standard model expectation, and upper limits on the production cross section are presented. Cross sections higher than 20 to 55 fb are excluded at 90\% confidence level (CL) for dark matter particles with masses ranging from 1 GeV to 1 TeV. Interpreting the findings in the context of a scalar interaction between dark matter particles and top quarks, we exclude a new region in the parameter space of the mass of the dark matter particle and the interaction scale. Assuming a dark matter particle with a mass of 100 GeV, the interaction scale is excluded at 90\% CL below 118 GeV, representing the most stringent limits to date. The limits on the interaction scale are also interpreted as the limits on the dark matter-nucleon scattering cross sections for the spin independent scalar operator we considered. For dark matter particles with masses below 6 GeV, dark matter-nucleon cross sections higher than $1-2\times10^{-42}$ $\mathrm{cm^{2}}$ are excluded with 90\% CL, representing the most stringent limits to date.