Search for an invisible scalar in tt final states at the LHC

We use the current tt experimental analysis to look for Dark Matter (DM) particles hidden in the final state. We present a phenomenological study where we successfully perform the reconstruction of a tt system in the presence of a scalar mediator Y 0 , that couples to both Standard Model (SM) and to DM particles. We use a MadGraph5_aMC@NLO simplified DM model, where signal samples of pp → ttY 0 are generated at the Large Hadron Collider (LHC) with both Charge-Parity (CP) -even and CP-odd couplings of Y 0 to the top quarks. Different mass scales for the Y 0 mediator are considered, from the low mass region ( 0 GeV) to masses close to the Higgs boson mass (125 GeV). The dileptonic final states of the tt system were used in our analysis. The reconstruction of the tt system is done with a kinematic fit, without reconstructing the mediator. All relevant SM backgrounds for the dileptonic tt search at the LHC are considered. Furthermore, CP angular observables were used to probe the CP-nature of the coupling between the mediator and top-quarks, which allowed to set confidence level (CL) limits for those Yukawa couplings as a function of the mediator mass.