Search for single production of vector-like quarks decaying into a Wb final state at $\sqrt{s}$ = 13 TeV with the ATLAS detector

The ΛCDM model of cosmology in combination with the Standard Model of particle physics (SM) provides the best understanding of the universe at both galactic and sub-atomic scales. The gravitational force is explained by Einstein's theory of relativity, which is the basis of the ΛCDM model. The measurements from the Planck Collaboration show that the universe comprises of about 70% dark energy, 25% dark matter, and 5% ordinary matter. The Standard Model of particle physics is a theory that gives a description of the fundamental building blocks of ordinary matter and the interactions among them namely the electroweak and the strong forces. The predictions of this theory have been tested and proven with high precision. A theory that only describes 5% of the universe may seem inconsequential, but the Standard Model can provide an explanation for the mechanism that makes the stars shine, all 1 × 10${}^{24}$ of them. Till date, there exists very limited understanding of both dark matter and dark energy. Dark matter is said to interact using gravitational forces but not electromagnetic forces. Dark energy is the form of energy that is driving the accelerated expansion of the universe. The discovery of the Higgs boson in 2012 by both the ATLAS and CMS collaborations using high energy proton-proton collisions at the Large Hadron Collider (LHC) ensured that all particles predicted by the SM were experimentally discovered. In the subsequent years, the Higgs boson and other SM particles have been studied in great detail using the data collected during the Run 1 and Run 2 of the LHC. The most precise measurement of the Higgs boson mass is (125.11 ± 0.11) GeV which was performed by the ATLAS collaboration. On the contrary, the quantum corrections for the predicted Higgs mass due to the interactions of the Higgs boson and other SM particles are proportional to the next scale of new physics, which is the Planck scale (1 × 10${}^{19}$ GeV) according to the current theories. Thus, there exists a significant discrepancy among the predicted and measured value of the Higgs boson mass. This problem is called the Higgs mass hierarchy problem. Theorists have proposed numerous theories that solve this problem by cancelling these corrections arising from the interactions of new particles predicted in these theories and the Higgs boson. The high center-of-mass energies of the proton-proton collisions at the LHC were the perfect toolkit to probe the validity of these Beyond Standard Model (BSM) theories. The 13 TeV center-of-mass energy used in the Run 2 of the LHC from 2015 to 2018 provided an opportunity to study unexplored high-energy domains. The new physics program of the LHC mainly comprised of experimental searches of hypothesised particles using the collision data gathered by the ATLAS and CMS detectors. Predictions from theories like Supersymmetry, Composite Higgs models, Little Higgs models were extensively tested. Premise and organisation of thesis One such prediction of various BSM theories is the existence of particles called Vector-like quarks, whose properties slightly differ from Standard Model quarks. Unlike SM particles, these particles' mass does not arise due to the Higgs mechanism. This ensures that Higgs measurements do not constrain the predictions concerning vector-like quarks. This thesis uses Run 2 LHC collision data collected by the ATLAS detector to search for the existence of vector-like quarks. These particles can be produced both via single production and pair production. In the phenomenological predictions, the single production has a higher production cross-section compared to pair production at higher energies. Thus, this thesis focusses on the single production search. There are several types of vector-like quarks that are hypothesised in theories. Vector-like quarks predominantly couple with third generation SM quarks in association with SM bosons. Among the various decay modes, the decay mode involving a W-boson and a b-quark was studied in this thesis. This choice was motivated by the relative high branching fraction of vector-like quarks to this decay mode. While studying decay modes in LHC collisions, only the the final decay mode can be directly accessed. For example, rather than measuring the W-boson, the decay products of a W-boson are used. For this thesis, both the leptonic and hadronic decay modes were probed. The leptonic decay mode was studied using the 2015-2016 collision dataset whereas the hadronic decay mode was investigated using the full Run 2 collision dataset collected from 2015-2018. The search for vector-like quarks performed in this thesis using the Wb hadronic decay mode is the first of its kind. This thesis is divided into three parts. The Part I introduces all general concepts, tools, and methodologies that have been used for this thesis. The Part II details the analysis and results of the search using the leptonic decay mode with 2015-2016 data. The work for this part resulted in a publication by the ATLAS collaboration. The Part III describes the analysis and results of the search using the hadronic decay mode with 2015-2018 data. This work is currently in review by the ATLAS collaboration for an upcoming publication.