Search for Dark Matter produced in association with hadronically decaying Standard Model bosons using $36~\textrm{fb}^{-1}$ of $pp$ collisions at the ATLAS detector

Dark Matter composes a significant part of the observable Universe. Despite a solid cosmological evidence, its nature, properties and interaction with the Standard Model sector is still to be unraveled. A wide range of different dark matter models beyond the Standard Model can be probed at the Large Hadron Collider. Two searches for the dark matter particles produced in association with the hadronically decaying Standard Model vector bosons and Higgs boson $h$, decaying to a $b\bar{b}$ pair, are presented. Both searches are performed using $36~\textrm{fb}^{-1}$ of the proton-proton collision data at $\sqrt{s} = 13~\textrm{TeV}$ collected with the ATLAS detector in 2015 and 2016. In preparation for the interpretation of the final search results, the performance of the $E_{\mathrm{T}}^{\mathrm{miss}}$ triggers used in both analyses is studied. The scale factors are developed to correct for the discrepancy between the $E_{\mathrm{T}}^{\mathrm{miss}}$ trigger efficiency in data and Monte Carlo simulation. No significant excess in the observed data over the Standard Model predictions is observed in both searches. The upper limits on the production cross-sections of $W/Z+E_{\mathrm{T}}^{\mathrm{miss}}$ and $h+E_{\mathrm{T}}^{\mathrm{miss}}$ events times the branching ratios of $W/Z \rightarrow q\bar{q}$ and $h \rightarrow b\bar{b}$ decays for the simplified vector mediator and $Z'-$2HDM model scenarios are set at 95% confidence level. The limits are significantly improved relative to the previous results using $3.2~\textrm{fb}^{-1}$ of data. Additionally, the model-independent representation is introduced for the first time for both analyses. The generic upper limits on the visible cross-section of $W/Z+E_{\mathrm{T}}^{\mathrm{miss}}$ and $h+E_{\mathrm{T}}^{\mathrm{miss}}$ events at 95% confidence level are presented in separate regions of the $E_{\mathrm{T}}^{\mathrm{miss}}$ variable at the detector- and particle-levels.