Impact of loosening the requirement on missing transverse momentum in tt+DMsearches with the ATLAS experiment

Abstract: Dark matter is required in the universe to explain certain phenomenona, like the dynamics of stars and temperature fluctuations of the cosmic microwave background. This thesis focuses on the production of dark matter in the ATLAS experiment at the Large Hadron Collider, where the dark matter is detected through momentum conservation. The models of interest are simplified dark matter models consisting of a pair of top quarks produced together with a mediator. It is the mediator, scalar or pseudoscalar having a mass of 20 GeV  that decays to dark matter particles each having a mass of 1 GeV. The final state considered in this thesis consists of jets, one lepton and missing transverse momentum.  The coupling constants between the mediator and the top quarks and the mediator and the dark matter particles is set to 1.  The goal of the work presented in this thesis is  to improve the searches for dark matter of dark matter, by studying the distributions of variables that can separate the signal from various background processes. This is done by loosening the requirement on missing transverse momentum in the dark matter searches from 230 GeV to 70 GeV and study the low missing transverse momenta samples. This thesis then finds that the low missing transverse momentum scalar sample has the highest yield of all samples studied. Moreover, the transverse mass selection efficiency for the low missing transverse momentum sample is found to be low and needs to be improved. The last conclusion presented is that the azimuthal angle between the lepton and the missing transverse momentum can be used to differentiate the mediator types in both the low and high missing transverse momentum samples.