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

From the discovery of the electron in 1897 by J.J Thomson to the Higgs boson in 2012 at the LHC, a plethora of elementary particles have been detected by various experiments. The Standard Model (SM) of particle physics has been enormously successful in describing all the observed elementary particles and explaining the interactions between them at the most fundamental scientific level. However, it is clear that SM alone is insufficient to answer many open questions in modern physics, such as the presence dark matter (DM) and dark energy (DE) in our universe. Ordinary matter, observed so far by various measurements, accounts for only about 5% of the energy density of the universe, while a large fraction is in the form of DM (27%) and DE (68%). The SM alone is insufficient to explain the existence of DM or DE, hence Beyond Standard Model (BSM) theories are postulated to explain the dark sector. One such set of BSM models are the so-called Higgs portal models, where the DM interacts with the SM particles through the Higgs sector. Various types of experiments are conducted around the world to search for DM and DE, and one of them is the Large Hadron Collider (LHC) in Geneva, Switzerland. The LHC is a particle accelerator that collides high-energy protons and data from the pp collisions are recorded by four large detectors, the largest being the ATLAS detector. This thesis describes the search for DM and DE with the pp collision data recorded by the ATLAS detector and the results are interpreted in terms of certain DM and DE models. No DM or DE particles were discovered, hence constraints are put on the DM and DE model parameters.