Search for a light vector dark matter particle in the tt final state at the LHC

Abstract

The dark matter (DM) problem can be addressed by the addition of a dark sector composed of viable DM particle candidates to the Standard Model (SM), requiring a DM boson to mediate the interactions between the dark and SM sectors. Employing a simplified DM model - the DMsimp model - in the work of this dissertation, we present a phenomenological study where we successfully perform the kinematic reconstruction of the system tt in the presence of a pure vector DM mediator, Y1+ , described by the DMsimp model, that only couples to the top quarks of the SM. Events involving the dileptonic channel of this final state, ttY1+ , were generated for the Large Hadron Collider using the Monte Carlo generator MADGRAPH5_AMC@NLO for different DM mediator mass scales, from the low-mass region (∼ 0 GeV) to the Higgs boson mass (125 GeV), and their response at the ATLAS detector were simulated. The reconstruction of the tt system is done with a kinematic fit for the dileptonic final state of the tt system, without reconstructing the mediator. All relevant SM backgrounds for the dileptonic tt channel search at the LHC are considered. Lastly, exclusion confidence level limits are set on the pure vector DM mediator couplings with the top quarks for all generated mediator masses using angular observables to probe how well they discern the ttY1+ signal from the SM background. Exclusion confidence level limits are also set on the pure scalar and pure pseudo-scalar DM mediator couplings with the top quarks to assess the exclusion of spin 0 DM mediators with well-defined CP quantum numbers when the SM plus a pure vector DM mediator is assumed as the null hypothesis.