Abstract
A search is presented for particle dark matter produced in association with a pair of top quarks in pp collisions at a centre-of-mass energy of sqrt(s) = 8 TeV. The data were collected with the CMS detector at the LHC and correspond to an integrated luminosity of 19.7 inverse femtobarns. This search requires the presence of one lepton, multiple jets, and large missing transverse energy. No excess of events is found above the SM expectation, and upper limits are derived on the production cross section. Interpreting the findings in the context of a scalar contact interaction between fermionic dark matter particles and top quarks, lower limits on the interaction scale are set. These limits are also interpreted in terms of the dark matter-nucleon scattering cross sections for the spin-independent scalar operator and they complement direct searches for dark matter particles in the low mass region.
Highlights
Background estimationStandard model backgrounds are estimated from simulation, with data-to-simulation scale factors (SF) applied to the dominant backgrounds from tt+jets and W+jets.Two control regions (CR) are defined to extract these SFs
We report on a search for the production of DM particles in association with a pair of top quarks, and consider only the scalar interaction
The choice of 90% confidence level (CL) is made in order to allow direct comparisons with related limits from astrophysical observations
Summary
The central feature of the CMS apparatus is a superconducting solenoid of 6 m internal diameter, providing a magnetic field of 3.8 T. Within the field volume are a silicon pixel and strip tracker, a lead tungstate crystal electromagnetic calorimeter (ECAL), and a brass and scintillator hadron calorimeter (HCAL), each composed of a barrel and two endcap sections. Muons are measured in gas-ionization detectors embedded in the steel flux-return yoke outside the solenoid. Extensive forward calorimetry complements the coverage provided by the barrel and endcap detectors. A more detailed description of the CMS detector, together with a definition of the coordinate system used and the relevant kinematic variables, can be found in ref. A more detailed description of the CMS detector, together with a definition of the coordinate system used and the relevant kinematic variables, can be found in ref. [13]
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