Search for an invisible scalar in $t \bar{t}$ final states at the LHC

TL;DR

This study searches for an invisible scalar mediator in top-antitop events at the LHC, using a phenomenological approach to reconstruct the system and probe CP properties, setting limits on couplings across various mediator masses.

Contribution

It introduces a method to reconstruct $tar{t}$ systems with a scalar mediator and uses CP angular observables to constrain mediator-top couplings at the LHC.

Findings

Reconstructed $tar{t}$ system with a scalar mediator using kinematic fit.

Set confidence level limits on Yukawa couplings as a function of mediator mass.

Analyzed CP properties of the mediator-top coupling.

Abstract

We use the current $t\bar t$ 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 $t\bar{t}$ system in the presence of a scalar mediator $Y_0$, that couples to both Standard Model (SM) and to DM particles. We use a \texttt{MadGraph5\_aMC@NLO} simplified DM model, where signal samples of $pp \rightarrow t\bar{t}Y_0$ 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 ($\sim$ 0~GeV) to masses close to the Higgs boson mass (125~GeV). The dileptonic final states of the $t\bar{t}$ system were used in our analysis. The reconstruction of the $t\bar{t}$ system is done with a kinematic fit, without reconstructing the mediator. All relevant SM backgrounds for the dileptonic $t\bar{t}$ 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.