Search for Dark Matter produced in association with a Higgs boson decaying to $b\bar{b}$ using $36~\text{fb}^{-1}$ of $pp$ collisions at $\sqrt{s}=13~\text{TeV}$ with the ATLAS Detector

Cosmological and astrophysical observations indicate the existence of Dark Matter. Many beyond Standard Model theories predict associated production of Dark Matter particles with a Higgs boson at energies that can be probed at collider experiments. The signature of the presented analysis consists of a $b\bar{b}$ pair from a Higgs boson decay recoiling against missing transverse momentum from Dark Matter particles, $h(\rightarrow b\bar{b})+E_{T}^{miss}$. The presented $h(\rightarrow b\bar{b})+E_{T}^{miss}$ search is performed using proton-proton collisions at a centre-of-mass energy of $13~\text{TeV}$ with an integrated luminosity of $36.1~\text{fb}^{-1}$ recorded with the ATLAS detector at the Large Hadron Collider. Several new refined event selections were developed, which increase considerably the experimental sensitivity of the search to beyond Standard Model theories resulting in a $h(\rightarrow b\bar{b})+E_{T}^{miss}$ signature. Discovery significance along with its gains from the new refined event selections are presented. The observed data are found to be in agreement with the Standard Model predictions. Upper limits on the production cross-section of $h+E_{T}^{miss}$ events times the $h\rightarrow b\bar{b}$ branching ratio in a two-Higgs-doublet model with an extra $Z^{\prime}$ boson scenario are presented at $95\%$ confidence level. The improvement of the limits from the new refined event selections is quantified. A new topological algorithm is suggested for future analyses and its associated increase in experimental sensitivity is also presented.