Search for dark matter produced in association with a Standard Model Higgs boson decaying to b-quarks with the ATLAS detector

The Standard Model (SM) of particle physics is one of the biggest triumphs of modern physics. The SM has been immensely successful in explaining the new elementary particles and how they interact with each other. A plethora of experiments have been conducted to validate the SM predictions, and so far the SM predictions and experimental observations are in good agreement, the most recent evidence being the discovery of Higgs Boson at the LHC in 2012. However, SM alone is insufficient to answer many open questions in modern physics, such as the presence of dark matter (DM) and dark energy (DE) in our universe. Ordinary matter, observed so far by various experiments, accounts for only about 5% of the energy density of the universe, while a large fraction is in the form of DM ($\sim$27%) and DE ($\sim$68%). While the nature of DM is still unknown, one of the leading hypotheses suggests that it consists of Weakly Interacting Massive Particles (WIMP). All evidence point to the interaction between DM and SM to be very weak. DM searches are being pursued in collider experiments alongside direct and indirect detection experiments. In this thesis, a search for dark matter candidates produced in association with a Standard Model Higgs boson decaying to two $b$-jets is presented. The search uses a dataset of $pp$ collisions at $\sqrt{s}=$ 13 TeV corresponding to an integrated luminosity of 139 fb$^{-1}$, recorded by the ATLAS detector. The results are interpreted in the context of the Two-Higgs Doublet Model (2HDM) with an additional vector or pseudoscalar mediator. The 2HDM is connected to the so-called Higgs portal models, in which DM particles interact with the SM particles only through their couplings with the Higgs sector. The analysis did not discover any DM particles and constraints are put on the model parameters. Some parts of the benchmark DM model phase-space are excluded and improvements are observed compared to previous results. Another limitation of the SM is its inability to explain the accelerated expansion of the universe. One possible explanation in the context of a homogeneous and isotropic universe is the mysterious DE, which is a repulsive force that counteracts the gravitational pull. This thesis describes the search for DE in the $t\bar{t}$+$E_{\rm T}^{\rm miss}$ final state with a dataset of $pp$ collisions at $\sqrt{s}=$ 13 TeV corresponding to the integrated luminosity of 36.1 fb$^{-1}$ recorded by the ATLAS detector. The results are based on a reinterpretation of the search for supersymmetric partners of top-quark to constrain conformal couplings of DE to SM matter. No DE particles were discovered and exclusion constraints are put on the DE model parameters.