Search for Dark Matter Produced in Association with a Higgs Boson decaying to $b\bar{b}$ at $\sqrt{s}= 13\,$TeV with the ATLAS Detector using 79.8$\,$fb$^{-1}$ of proton-proton collision data

A search for dark matter produced in association with a Higgs boson decaying to $b$-quarks is performed using $pp$ collisions at a centre-of-mass energy of $\sqrt{s}= 13\,$TeV. The dataset has an integrated luminosity of 79.8$\,$fb$^{-1}$ and was recorded with the ATLAS detector at the Large Hadron Collider. Selected collision events comprise large missing transverse momentum and either two $b$-tagged small radius jets or a single large radius jet containing two $b$-tagged subjets. The reconstruction of these subjets is based on a jet algorithm that uses inner detector tracks as inputs and a radius parameter that decreases as the subjet transverse momentum increases. The observed data are in agreement with the Standard Model predictions. The results are interpreted in the context of a simplified model ($Z^{\prime}$-2HDM) which describes the interaction of dark matter and Standard Model particles via new heavy mediator particles. Masses of the $Z^{\prime}$ are excluded up to $2.8\,$TeV depending on the choices for other model parameters.