Search for Dark Matter in association with a top quark and a $W$-boson in $\sqrt{s}$ = 13 TeV pp collisions at the ATLAS detector

Abstract

The Standard Model (SM) is the only model in particle physics containing all observed elementary particles and their interactions. Over the years, its predictions were tested and observed in a number of experiments. However, there are still observations that can not be explained by the SM with one of the most prominent ones being the existence of Dark Matter (DM). While the existence of DM was first theorised through astronomical observations, extensions of the SM allow for a search of DM at the Large Hadron Collider (LHC) as well. Since DM can not be detected directly, final states analysing its existence at the LHC are always designed around high missing transverse energy. This thesis presents a search for DM with data collected by the ATLAS detector in the years 2015-2018 corresponding to an integrated luminosity of 139~$\mathrm{fb}^{-1}$ at a center of mass energy of 13 TeV. The analysis is based on an extended two Higgs doublet model (2HDM+a) where a pseudo-scalar mediator allows the production of DM in the final state. In addition to the DM, a top quark and a $W$-boson-boson are produced in the final state. This thesis mainly focuses on a final state with zero leptons which results in both the $W$-boson-boson and the top quark being required to decay hadronically. In addition to that, the $W$-boson-boson can be expected to have a high momentum in the signal process. Therefore, a dedicated identification algorithm using large-radius jets is employed to select events with at least one hadronically decaying $W$-boson-boson. This allows for a strong distinction against SM background events. In this analysis, several different signal processes are considered, since different parameters can affect the cross section and the distributions of the signal processes. These parameters include different values for the mass of the mediator $a$, the mass of the heavy Higgs boson $H$ and $\tan\beta$. Since no significant excess was found in the signal regions when comparing SM prediction to data, upper exclusion limits on the cross section of the signal processes were calculated with a 95% confidence level (CL). These limits are presented in three different model parameter planes. One such plane presents the upper limits for different values of the mass of the mediator, $m_a$, and the mass of the heavy Higgs boson, $m_H$, while keeping $\tan\beta = 1$. The second and third plane present the upper limits for different values of the mass of the heavy Higgs boson, $m_H$, and $\tan\beta$ while the mass of the mediator $a$ stays constant at $m_a=$ 250~GeV and $m_a=$ 150~GeV. In addition to the upper limits for signal processes with one top quark in the final state, this thesis also provides an upper exclusion limit where this signal process is combined with a process that includes two top quarks in the final state